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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2018	Mar. 05, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	SYBX
Entity Registrant Name	SYNLOGIC, INC.
Entity Central Index Key	0001527599
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		25,400,495
Entity Public Float			$ 174.8

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 11,252	$ 58,440
Short-term marketable securities	111,477	28,585
Prepaid expenses and other current assets	1,609	1,564
Total current assets	124,338	88,589
Property and equipment, net	14,841	9,783
Restricted cash	1,097	1,097
Other assets	64	230
Total assets	140,340	99,699
Current liabilities:
Accounts payable	2,380	2,679
Accrued expenses	5,034	4,823
Deferred revenue	268	444
Deferred rent	393	656
Capital lease obligations	266	425
Total current liabilities	8,341	9,027
Long-term liabilities:
Deferred revenue, net of current portion		668
Deferred rent, net of current portion	7,691	4,500
Capital lease obligations, net of current portion	210	466
Total long-term liabilities	7,901	5,634
Commitments and contingencies (Note 18)
Stockholders' Equity
Preferred stock, $0.001 par value 5,000,000 shares authorized, none issued and outstanding as of December 31, 2018 and December 31, 2017
Common stock, $0.001 par value 250,000,000 shares authorized as of December 31, 2018 and December 31, 2017. 25,401,479 shares issued and outstanding as of December 31, 2018 and 16,272,617 shares issued and outstanding as of December 31, 2017.	25	16
Additional paid-in capital	243,903	156,685
Accumulated other comprehensive loss	(65)	(9)
Accumulated deficit	(119,765)	(71,654)
Total stockholders' equity	124,098	85,038
Total liabilities and stockholders' equity	$ 140,340	$ 99,699

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, Shares Authorized	5,000,000	5,000,000
Preferred stock, Shares Issued	0	0
Preferred stock, Shares Outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, authorized	250,000,000	250,000,000
Common stock, Issued	25,401,479	16,272,617
Common stock, outstanding	25,401,479	16,272,617

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Revenue	$ 2,520	$ 2,444
Operating expenses:
Research and development	38,034	30,341
General and administrative	15,716	12,927
Total operating expenses	53,750	43,268
Loss from operations	(51,230)	(40,824)
Other income (expense):
Interest and investment income	2,843	504
Interest expense	(43)	(57)
Other expense	(5)
Other income (expense), net	2,795	447
Net loss	$ (48,435)	$ (40,377)
Net loss per share attributable to common shareholders - basic and diluted	$ (2.03)	$ (6.00)
Weighted-average common shares used in computing net loss per share attributable to common shareholders - basic and diluted	23,882,685	6,724,641
Comprehensive loss:
Net loss	$ (48,435)	$ (40,377)
Net unrealized losses on marketable securities	(56)	(9)
Comprehensive loss	$ (48,491)	$ (40,386)

Consolidated Statements of Contingently Redeemable Preferred Equity and Stockholders' Equity - USD ($) $ in Thousands	Total	Common units	Common Shares $0.0001 Par	Common Shares $0.001 Par	Additional Paid-in Capital	Unrealized Gain/(Loss) on Securities	Accumulated Deficit	Contingently redeemable Series A preferred stock	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Series C Convertible Preferred Stock	Contingently redeemable Class A preferred units	Class A preferred units	Class B preferred units
Balance at Dec. 31, 2016												$ 5,000
Balance (in units) at Dec. 31, 2016												781,693
Balance (in units) at Dec. 31, 2016													3,922,027	1,029,850
Balance at Dec. 31, 2016													$ 25,548	$ 13,611
Balance at Dec. 31, 2016	$ 8,503						$ (31,248)
Sale of preferred units, net of issuance costs	26,648													$ 26,648
Sale of preferred units, net of issuance costs (in units)														1,971,717
Issuance of common stock for license agreement	1,750	$ 1,750
Exchange of preferred and common units into preferred and common stock								$ 5,000				$ 5,000
Exchange of preferred and common units into preferred and common stock, Units								781,693				781,693
Exchange of preferred and common units into preferred and common stock, (in units)													(3,922,027)	(3,001,567)
Exchange of preferred and common units into preferred and common stock													$ (25,548)	$ (40,259)
Exchange of preferred and common units into preferred and common stock									$ 25,548	$ 40,259
Exchange of preferred and common units into preferred and common stock, shares			2,020,367						3,922,027	3,001,567
Exchange of preferred and common units into preferred and common stock					$ 2,342
Sale of preferred stock, net of issuance costs	40,433										$ 40,433
Sale of preferred stock, net of issuance costs (in units)											2,882,679
Convertible preferred stock and contingently redeemable preferred stock exchanged for common stock								$ (5,000)
Convertible preferred stock and contingently redeemable preferred stock exchanged for common stock, Units								(781,693)
Convertible preferred stock and contingently redeemable preferred stock exchanged for common stock									$ (25,548)	$ (40,259)	$ (40,433)
Convertible preferred stock and contingently redeemable preferred stock exchanged for common stock									(3,922,027)	(3,001,567)	(2,882,679)
Convertible preferred stock and contingently redeemable preferred stock exchanged for common stock	5,000			$ 10	111,230
Convertible preferred stock and contingently redeemable preferred stock exchanged for common stock, shares				10,587,966
Common stock ($.0001 par) exchanged for common stock ($.001 par)				$ 3	(3)
Common stock ($.0001 par) exchanged for common stock ($.001 par), shares			(2,714,694)	2,714,694
Issuance of common stock in the Merger	40,433			$ 3	40,430
Issuance of common stock in the Merger, shares				2,979,836
Exercise of stock options	5				5
Exercise of stock options, shares				386
Issuance of restricted stock, shares			697,292	2,884
Cancellation of restricted stock			(2,965)	(13,149)
Equity-based compensation expense	2,652				2,652
Effect of adoption of ASU \| ASU 2016-09					29		(29)
Unrealized gain/(loss) on securities	(9)					$ (9)
Net loss	(40,377)						(40,377)
Balance at Dec. 31, 2017	$ 85,038			$ 16	156,685	(9)	(71,654)
Balance (in units) at Dec. 31, 2016		1,847,615
Balance at Dec. 31, 2016		$ 592
Issuance of common stock for license agreement (in units)		179,996
Repurchase of founders' units		(7,244)
Exchange of common units into preferred and common stock, (in units)		(2,020,367)
Exchange of common units into preferred and common stock, value		$ (2,342)
Balance (in Shares) at Dec. 31, 2017	16,272,617			16,272,617
Sale of common stock	$ 82,666			$ 9	82,657
Sale of common stock, shares				9,179,500
Exercise of stock options	244				244
Exercise of stock options, shares				19,830
Cancellation of restricted stock				(70,468)
Equity-based compensation expense	4,317				4,317
Effect of adoption of ASU \| ASU 2014-09 (ASC 606)	324						324
Unrealized gain/(loss) on securities	(56)					(56)
Net loss	(48,435)						(48,435)
Balance at Dec. 31, 2018	$ 124,098			$ 25	$ 243,903	$ (65)	$ (119,765)
Balance (in Shares) at Dec. 31, 2018	25,401,479			25,401,479

Consolidated Statements of Contingently Redeemable Preferred Equity and Stockholders' Equity (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares
Common shares, par value \| $ / shares	$ 0.001
Exchanged common shares, par value \| $ / shares	$ 0.0001
Class B Preferred Units
Issuance costs \| $	$ 18
Series C Convertible Preferred Stock
Issuance costs \| $	$ 1,567

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (48,435)	$ (40,377)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	2,421	2,310
Loss on disposal of property and equipment	8	5
Equity-based compensation expense	4,317	2,652
Common shares issued for license acquisition		1,750
Accretion/amortization of investment securities	(1,401)	(6)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(45)	(87)
Accounts payable and accrued expenses	(257)	4,071
Deferred revenue	(520)	(444)
Deferred rent	1,274	(1,121)
Other assets	168	192
Net cash, cash equivalents and restricted cash used in operating activities	(42,470)	(31,055)
Cash flows from investing activities:
Net assets acquired in reverse merger, net of transaction costs		40,433
Purchases of marketable securities	(172,887)	(51,438)
Proceeds from maturity of marketable securities	91,340	22,850
Proceeds from sale of property and equipment		11
Purchases of property and equipment	(5,654)	(2,578)
Net cash, cash equivalents and restricted cash (used in) provided by investing activities	(87,201)	9,278
Cash flows from financing activities:
Payments on capital lease obligations	(427)	(408)
Proceeds from exercise of stock options	244	5
Proceeds from sale of common stock, net of issuance costs	82,666
Proceeds from sale of convertible preferred stock, net of issuance costs		40,433
Proceeds from sale of preferred units, net of issuance costs		26,648
Net cash, cash equivalents and restricted cash provided by financing activities	82,483	66,678
Net (decrease) increase in cash, cash equivalents and restricted cash	(47,188)	44,901
Cash, cash equivalents and restricted cash at beginning of period	59,537	14,636
Cash, cash equivalents and restricted cash at end of period	12,349	59,537
Supplemental disclosure of non-cash investing activities:
Landlord funded allowance for tenant improvements	1,654	4,961
Property and equipment purchases included in accounts payable and accrued expenses	169	147
Supplemental disclosure of non-cash financing activities:
Cash paid for interest	43	35
Purchase under capital lease	$ 12	918
Prior period adjustment related to the adoption of ASU 2016-09		$ 29

Nature of Business

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Nature of Business

(1)	Nature of Business

Organization

Synlogic, Inc., together with its wholly owned and consolidated subsidiaries (“Synlogic” or the “Company”), is a clinical-stage biopharmaceutical company focused on advancing its drug discovery and development platform for Synthetic Biotic™ medicines. Synthetic Biotic medicines are generated from Synlogic’s proprietary drug discovery and development platform applying the principles and tools of synthetic biology to engineer beneficial microbes to perform or deliver critical therapeutic functions to treat metabolic and inflammatory diseases and cancer. As living medicines, Synthetic Biotic medicines can be designed to sense a local disease context within a patient’s body and to respond by metabolizing a toxic substance, compensating for missing or damaged metabolic pathways in patients, or by delivering combinations of therapeutic factors. Synlogic’s goal is to lead in the discovery and development of Synthetic Biotic therapies as living medicines capable of robust and precise pathway complementation and delivery of therapeutic benefit. Since incorporation, the Company has devoted substantially all of its efforts to the research and development of its product candidates.

Synlogic, Inc. (“Private Synlogic” when referred to prior to the Merger (as defined below)) was founded and began operations on March 14, 2014, as TMC Therapeutics, Inc., located in Cambridge, Massachusetts. On July 15, 2014, TMC Therapeutics, Inc. changed its name to Synlogic, Inc. On July 2, 2015, the common and preferred stockholders of Private Synlogic executed the Synlogic, LLC Contribution Agreement (the “Contribution Agreement”), pursuant to which such common and preferred stockholders contributed such stockholders’ equity interests in Private Synlogic in exchange for common and preferred units in a newly formed parent company named Synlogic, LLC. In addition, Synlogic IBDCo, Inc. (“IBDCo”) was formed as a subsidiary of Synlogic, LLC (the “2015 Reorganization”). In conjunction with the 2015 Reorganization, Private Synlogic entered into a license, option and merger agreement with AbbVie S.à.r.l. (“AbbVie”), for the development of treatments for inflammatory bowel disease (“IBD”).

In May 2017, Private Synlogic completed a reorganization (“2017 Reorganization”) pursuant to which Synlogic, LLC merged with and into Private Synlogic, with Private Synlogic continuing as the surviving corporation. Pursuant to the 2017 Reorganization, the common units and preferred units of Synlogic, LLC, together consisting of Class A preferred units, contingently redeemable Class A preferred units and Class B preferred units, were exchanged for common stock and preferred stock of Private Synlogic, respectively. Additionally, Private Synlogic issued equity awards under the Synlogic 2017 Stock Incentive Plan (“2017 Plan”) to replace the canceled incentive units pursuant to the termination of the Synlogic, LLC 2015 Equity Incentive Plan (“2015 LLC Plan”).

On August 28, 2017, Synlogic, Inc., formerly known as Mirna Therapeutics, Inc. (NASDAQ: MIRN) (“Mirna”), completed its business combination with Private Synlogic pursuant to the Agreement and Plan of Merger and Reorganization, dated as of May 15, 2017, by and among Mirna, Meerkat Merger Sub, Inc. (“Merger Sub”), and Private Synlogic (the “Merger Agreement”), pursuant to which Merger Sub merged with and into Private Synlogic, with Private Synlogic surviving as a wholly owned subsidiary of Mirna (the “Merger”). Immediately after completion of the Merger, Mirna changed its name to “Synlogic, Inc.” (NASDAQ: SYBX).

Risks and Uncertainties

At December 31, 2018, the Company had approximately $122.7 million in cash, cash equivalents, and marketable securities, approximately $1.1 million of restricted cash, and an accumulated deficit of approximately $119.8 million. Since its inception through December 31, 2018, the Company has primarily financed its operations through the issuance of preferred stock and units, the sale of its common stock, the AbbVie collaboration, and cash received in the Merger. In the absence of positive cash flows from operations, the Company is highly dependent on its ability to find additional sources of funding in the form of debt or equity financing. In January 2018, the Company sold shares of its common stock in a firm commitment, underwritten public offering and received $53.8 million in net proceeds from this offering, after underwriting discounts and commissions and other offering expenses. In April 2018, the Company sold shares of its common stock in a registered direct offering and received $28.9 million in net proceeds from this offering, after fees and other expenses. Management believes that the Company has sufficient cash to fund its operations through at least twelve months from the issuance of these financial statements.

As an early-stage company, the Company is subject to a number of risks common to other life science companies, including, but not limited to, raising additional capital, development by its competitors of new technological innovations, risk of failure in preclinical and clinical studies, safety and efficacy of its product candidates in clinical trials, the risk of relying on external parties such as contract research organizations (“CROs”) and contract manufacturing organizations (“CMOs”), the regulatory approval process, market acceptance of the Company’s products once approved, lack of marketing and sales history, dependence on key personnel and protection of proprietary technology. The Company’s therapeutic programs are currently pre-commercial, spanning discovery through early development and will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization of any product candidates. These efforts require significant amounts of additional capital, adequate personnel, infrastructure, and extensive compliance-reporting capabilities. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate revenue from product sales. The Company may never achieve profitability, and unless and until it does, it will continue to need to raise additional capital or obtain financing from other sources, such as strategic collaborations or partnerships.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

(2)	Summary of Significant Accounting Policies

Basis of Presentation

The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (“U.S.”) (“U.S. GAAP” or “GAAP”).

Principles of Consolidation

The accompanying consolidated financial statements include the accounts of Synlogic and its wholly owned subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation.

Use of Estimates

The preparation of financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, and related disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting period. On an on-going basis, the Company’s management evaluates its estimates, including those related to revenue recognition, income taxes including the valuation allowance for deferred tax assets, research and development accruals, accrued expenses, contingencies and equity-based compensation. The Company bases its estimates on historical experience and on various other assumptions that are believed to be reasonable, the results of which form the basis for making judgments about the carrying values of assets and liabilities. Actual results could differ from those estimates. Changes in estimates are reflected in reported results in the period in which they become known.

Cash Equivalents

The Company considers all highly liquid investment instruments with a remaining maturity when purchased of three months or less to be cash equivalents. Investments qualifying as cash equivalents primarily consist of money market funds and corporate debt securities. Cash equivalents are stated at cost plus accrued interest, which approximates fair value. The amount of cash equivalents included in cash and cash equivalents was approximately $0.3 million and $32.7 million at December 31, 2018 and 2017, respectively.

Concentration of Credit Risk

Financial instruments that potentially subject the Company to concentrations of credit risk include amounts held as cash, cash equivalents, marketable securities and restricted cash. The Company uses high quality, accredited financial institutions to maintain its balances, and accordingly, such funds are subject to minimal credit risk. The Company has not experienced any losses in such accounts and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held. The Company has no financial instruments with off-balance sheet risk of loss.

Restricted Cash

The Company held cash of approximately $1.0 million at December 31, 2018 in a letter of credit to secure its lease at the 301 Binney Street facility. In addition, the Company held cash of $50,000 at December 31, 2018 and 2017 in a separate restricted bank account as collateral for the Company’s credit cards. The Company has classified these deposits as long-term restricted cash on its balance sheet.

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the statement of financial position that sum to the total of the same such amounts shown in the statement of cash flows (in thousands).

	December 31,		December 31,
	2018		2017
Cash and cash equivalents	$	11,252	$	58,440
Restricted cash included in other long-term assets		1,097		1,097
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$	12,349	$	59,537

Fair Value

The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. Accounting Standards Codification (“ASC”) Topic 820, Fair Value Measurements and Disclosures, establishes a fair value hierarchy for those instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). The hierarchy consists of three levels:

•

Level 1 – Utilize observable inputs such as quoted prices in active markets for identical assets or liabilities;

•

Level 2 – Utilize data points that are either directly or indirectly observable, such as quoted prices, interest rates and yield curves;

•

Level 3 – Utilize unobservable data points in which there is little or no market data, which require the Company to develop its own assumptions for the asset or liability.

The Company evaluates transfers between levels at the end of each reporting period. There were no transfers of assets or liabilities between Level 1, Level 2 or Level 3 during the years ended December 31, 2018 and 2017.

Available-for-Sale Securities

The Company classifies all short-term investments with an original maturity when purchased of greater than three months as available-for-sale. Available-for-sale securities are carried at fair value, with the unrealized gains and losses reported in other comprehensive income (loss). The amortized cost of debt securities in this category is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization is included in interest and investment income. Realized gains and losses, and declines in value judged to be other than temporary on available-for-sale securities, are included in interest and investment income.

The cost of securities sold is based on the specific identification method. Interest and dividends on securities classified as available-for-sale are included in interest and investment income. To determine whether an other-than-temporary impairment exists, the Company considers whether it has the ability and intent to hold the investment until a market price recovery, and whether evidence indicating the recoverability of the cost of the investment outweighs evidence to the contrary.

Property and Equipment

Property and equipment, including leasehold improvements, are recorded at cost and depreciated over their estimated useful lives using the straight‑line method. Repairs and maintenance costs are expensed as incurred, whereas major improvements are capitalized as additions to property and equipment.

Depreciation begins at the time the asset is placed in service. Depreciation is provided over the following estimated useful lives:

Asset classification		Useful life
Computer and office equipment		3 years
Furniture and fixtures		5 years
Laboratory equipment		5 years
Leasehold improvements		Lesser of useful life or remaining lease term

Impairment of Long‑Lived Assets

Long‑lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. When such events occur, the Company compares the carrying amounts of the assets to their undiscounted expected future cash flows. If this comparison indicates that there is impairment, the amount of impairment is calculated as the difference between the carrying value and fair value of the asset. To date, no such impairments have been recognized.

Rent Expense

The Company’s leases for both the 301 Binney Street facility and the 200 Sidney Street facility in Cambridge, Massachusetts provide for a rent-free period as well as fixed increases in minimum annual rental payments. The total amount of rental payments due over the lease term is being charged to rent expense on a straight-line basis over the term of the lease. Tenant improvement allowances and other incentives are recorded as deferred rent and amortized as a reduction of periodic rent expense, over the term of the lease. Deferred rent consists of the difference between cash payments and the recognition of rent expense on a straight-line basis for the Company’s facilities. The Company began to accelerate the recognition of deferred rent on its 200 Sidney Street facility when it agreed to terminate the lease in July 2017.

Research and Development Costs

Costs incurred in the research and development of the Company’s product candidates are expensed as incurred. The Company defers and capitalizes nonrefundable advance payments made by the Company for research and development activities until the related goods are received or the related services are performed.

Research and development expenses are comprised of costs incurred in performing research and development activities, including salary and benefits, equity-based compensation expense, laboratory supplies and other direct expenses, facilities expenses, overhead expenses, contractual services and other outside expenses.

When third-party service providers’ billing terms do not coincide with the Company’s period-end, the Company is required to make estimates of its obligations to those third parties, including clinical trial costs, contractual services costs and costs for supply of its drug candidates, incurred in a given accounting period and record accruals at the end of the period. The Company bases its estimates on its knowledge of the research and development programs, services performed for the period and the expected duration of the third-party service contract, where applicable.

Revenue recognition

The Company generates revenue through a collaboration and license arrangement with a strategic partner for the development and commercialization of product candidates.

Effective January 1, 2018, the Company adopted ASC Topic 606, Revenue from Contracts with Customers (“ASC 606”) using the modified retrospective transition method. Refer to the Recently Adopted Accounting Pronouncements section below for additional information on the new standard and the impact to our results of operations. Under this method, results for reporting periods beginning after January 1, 2018 are presented under ASC 606, while prior period amounts are not adjusted and continue to be reported in accordance with ASC Topic 605, Revenue Recognition (“ASC 605”).

The Company evaluates collaboration agreements with respect to FASB ASC Topic 808, Collaborative Arrangements, considering the nature and contractual terms of the arrangement and the nature of its business operations to determine the classification of the transactions. When the Company is an active participant in the activity and exposed to significant risks and rewards dependent on the commercial success of the collaboration, it will record its transactions on a gross basis in the consolidated financial statements and describe the rights and obligations under the collaborative arrangement in the notes to the consolidated financial statements.

Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following five-step analysis: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step analysis to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied.

The Company may enter into collaboration agreements for research and development services, under which the Company may license certain rights to its product candidates to third parties. The terms of these arrangements typically include payment to the Company of one or more of the following: non-refundable, upfront license fees; reimbursement of certain costs; customer option exercise fees; development, regulatory and commercial milestone payments; and royalties on net sales of licensed products. Variable consideration is constrained until it is deemed not be at significant risk of reversal.

In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under each of its agreements for which the collaboration partner is also a customer, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. As part of the accounting for these arrangements, the Company must use significant judgment to determine: a) the number of performance obligations based on the determination under step (ii) above; b) the transaction price under step (iii) above; and c) the contract term and pattern of satisfaction of the performance obligations under step (v) above. The Company uses significant judgment to determine whether milestones or other variable consideration, except for royalties, should be included in the transaction price as described further below. The transaction price is allocated to the goods and services the Company expects to provide. The Company uses estimates to determine the timing of satisfaction of performance obligations, which may include the use of full time equivalent time as a measure of satisfaction of performance obligations.

Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company’s consolidated balance sheets. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current deferred revenue. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion.

Licenses of Intellectual Property

In assessing whether a promise or performance obligation is distinct from the other promises, the Company considers factors such as the research, manufacturing and commercialization capabilities of the customer and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the customer can benefit from a promise for its intended purpose without the receipt of the remaining promises, whether the value of the promise is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition.

Research and Development Services

If an arrangement is determined to contain a promise or obligation for the Company to perform research and development services, the Company must determine whether these services are distinct from the other promises in the arrangement. In assessing whether the services are distinct from the other promises, the Company considers the capabilities of the customer to perform these same services. In addition, the Company considers whether the customer can benefit from a promise for its intended purpose without the receipt of the remaining promise, whether the value of the promise is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For research and development services that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition.

Customer Options

If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, that is, the option to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. The Company allocates the transaction price to material rights based on an alternative approach when the goods or services are both (i) similar to the original goods and services in the contract and (ii) provided in accordance with the terms of the original contract. Under this alternative, the Company allocates the total amount of consideration expected to be received from the customer to the total goods or services expected to be provided to the customer. Amounts allocated to a material right are not recognized as revenue until the option is exercised and the performance obligation is satisfied.

Milestone Payments

At the inception of each arrangement that includes milestone payments, the Company evaluates whether a significant reversal of cumulative revenue provided in conjunction with achieving the milestones is probable and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant reversal of cumulative revenue would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. For other milestones, the Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant reversal of cumulative revenue would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment.

Royalties

For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements.

Contract Costs

The Company recognizes as an asset the incremental costs of obtaining a contract with a customer if the costs are expected to be recovered. As a practical expedient, the Company recognizes the incremental costs of obtaining a contract as an expense when incurred if the amortization period of the asset that we otherwise would have recognized is one year or less. To date, the Company has not incurred any incremental costs of obtaining a contract with a customer.

Equity‑Based Compensation

The Company measures equity-based compensation to employees and directors based on the grant date fair value of the awards and recognizes the associated expense in the financial statements over the requisite service period of the award, which is generally the vesting period.

Equity‑based compensation costs for nonemployee awards are recognized as services are provided, which is generally the vesting period, on a straight‑line basis. The measurement date for nonemployee awards is generally the date the performance of services required from the nonemployee is complete. The Company believes that the fair value of the equity is more reliably measurable than the fair value of the services rendered. The fair value of the award granted to a nonemployee is remeasured at each reporting date until performance is completed with any increase or decrease in fair value recorded as equity‑based compensation expense.

Prior to the Merger in August 2017, the Company’s Board of Directors determined the estimated per share fair market value of the common stock and common units at various dates considering contemporaneous valuations performed in accordance with the guidance outlined in the American Institute of Certified Public Accountants Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, or the Practice Aid. The fair market value of the common stock and common units was determined by the Board of Directors at each award grant date based on assumptions, each of which are subjective and generally require judgement and estimation by management, including results obtained from independent third‑party valuations, the Company’s financial position and historical financial performance, the status of technological developments within the Company’s product candidates, the composition and ability of the research and management team, an evaluation or benchmark of the Company’s competition, the business climate in the marketplace, the illiquid nature of the common stock and common units, arm’s length sales of the Company’s capital stock (including convertible preferred stock), the effect of the rights and preferences of the preferred stock, and the prospects of a liquidity event.

The fair value of each option was estimated on the date of grant or remeasurement using the Black‑Scholes option‑pricing model. Expected volatility for the Company’s common stock was determined based on an average of the historical volatility of a peer‑group of similar public companies. The expected term of options granted for employees was calculated using the simplified method, which represented the average of the contractual term of the option and the weighted-average vesting period of the option. The assumed dividend yield is based upon the Company’s expectation of not paying dividends in the foreseeable future. The risk‑free interest rate is based upon the U.S. Treasury yield curve commensurate with the expected term at the time of grant or remeasurement. Forfeitures are recognized as they occur as allowed under ASU 2016-09.

The Company’s Board of Directors estimated the threshold price for each incentive unit issued by Synlogic, LLC, which is the price at which an incentive unit would have had a liquidation value of zero, considering the fair value of the Company’s assets at the date of grant and performed an analysis to determine the per unit amount that a holder would have received upon a distribution event. In determining the fair value of its assets, the Company relied on independent third-party valuations, which take into account a variety of factors, including the Company’s financial position and historical financial performance, the status of technological developments within the Company’s products, the composition and ability of the research and management team, an evaluation or benchmark of the Company’s competition, the business climate in the marketplace, the illiquid nature of the common units and incentive units, arm’s-length sales of the Company’s equity, the effect of the rights and preferences of the preferred unit holders, and the prospects of a liquidity event, among others.

The fair value of each incentive unit award was estimated on the date of grant or remeasurement using the Black‑Scholes with barrier option‑pricing model. Assumptions utilized in the model for valuing the incentive units including expected volatility, dividend yield and risk-free interest rate were arrived at in the same manner as those utilized for the stock option model described above. Forfeitures are treated in the manner described above. Incentive units did not have an expiration date, thus, the expected term of incentive units granted was determined based on the probability‑weighted estimated term to a liquidity event.

The Company records the expense for equity grants subject to performance-based milestone vesting over the remaining service period when management determines that achievement of the milestone is probable. Management evaluates when the achievement of a performance-based milestone is probable based on the relative satisfaction of the performance conditions as of the reporting date.

The Company classifies equity-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified.

Income Taxes

The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial reporting and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes based upon the weight of available evidence, that it is more likely than not that all or a portion of deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies.

Uncertain tax positions represent tax positions for which reserves have been established. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more likely than not to be sustained, the tax position is then assessed to determine the amount of benefit to be recognized in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.

Net Loss Per Share

Basic net loss per share is computed using the weighted-average number of shares of common stock outstanding during the period. Diluted net loss per share is computed using the sum of the weighted-average number of shares of common stock outstanding during the period and if dilutive, the weighted-average number of potential shares of common stock, including unvested restricted common stock and outstanding stock options.

The Company computed basic and diluted net loss per shares using the two-class method, which gives effect to the impact of the outstanding participating securities. As the years ended December 31, 2018 and 2017 resulted in net losses attributable to common stockholders, there is no income allocation required under the two-class method or dilution attributed to weighted-average shares outstanding in the calculation of diluted net loss per share because the preferred stockholders do not participate in losses of the Company. Accordingly, for periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common stock are not assumed to have been issued if their effect is anti-dilutive.

As the 2017 Reorganization resulted in a one for one conversion of preferred units for preferred stock and common units for common stock, the conversion was not substantive for the purposes of this calculation and the weighted average was calculated as if outstanding equity was outstanding from the beginning of the period presented.

Additionally, at the Effective Time of the Merger, the Company issued shares of its common stock to Private Synlogic stockholders, at the Exchange Ratio of 0.5532 shares of common stock, after taking into account the Reverse Stock Split, in exchange for each share of Private Synlogic preferred and common stock outstanding immediately prior to the Merger. The Exchange Ratio was calculated by a formula pursuant to the Merger Agreement. For the purposes of calculating net loss per share, the Exchange Ratio was applied retroactively to all periods presented.

Segment Information

Operating segments are defined as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company operates in one operating segment: discovery and development of synthetic biology therapeutics for the treatment of rare, infectious and other diseases. The Company’s chief executive officer, as chief operating decision maker, manages and allocates resources to the operations of the Company on a total company basis. All of the Company’s equipment, leasehold improvements and other fixed assets are physically located within the United States, and all agreements with its partners are denominated in U.S. dollars, except where noted.

Recently Adopted Accounting Pronouncements

Revenue Recognition

In May 2014, the FASB, issued Accounting Standards Update, (ASU), No. 2014-09, which amends the guidance for accounting for revenue from contracts with customers. This ASU supersedes the revenue recognition requirements in ASC 605 and creates ASC 606. In 2015 and 2017, the FASB issued additional ASUs related to ASC 606 that delayed the effective date of the guidance and clarified various aspects of the new revenue guidance, including principal versus agent considerations, identifying performance obligations, and licensing, and they include other improvements and practical expedients.

Effective January 1, 2018, the Company adopted ASC 606 using the modified retrospective transition method. Under this method, results for reporting periods beginning after January 1, 2018 are presented under ASC 606, while prior period amounts are not adjusted and continue to be reported in accordance with ASC 605. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance and financial instruments. As a result of adopting ASC 606 on January 1, 2018, the Company recorded a cumulative-effect decrease to opening accumulated deficit of $0.3 million as of January 1, 2018 and a corresponding decrease to deferred revenue. Total revenue recorded in the twelve months ended December 31, 2018 under ASC 606 was $2.5 million, as compared to $2.4 million that would have been recorded under ASC 605. Deferred revenue as of December 31, 2018 was $0.3 million under ASC 606, as compared to a balance of $0.7 million which would have resulted under ASC 605.

The most significant changes relate to the Company’s revenue recognition pattern for the AbbVie collaboration and the accounting for milestone payments. Under ASC 605, the Company was recognizing the revenue allocated to each unit of accounting on a straight line basis over the period the Company is expected to complete its obligations. Under ASC 606, the Company is recognizing the revenue allocated to each performance obligation measuring progress using an input method over the period the Company is expected to complete each performance obligation. Under ASC 605, the Company recognized revenue related to milestone payments as the milestone was achieved, using the milestone method. Under ASC 606, the Company determined that the milestones at the beginning of certain research and development phases represent a 90-day contract with daily customer renewal options for the Company’s continued research and development services. As a result, revenue from these milestones is recognized over a performance obligation consisting of the next phase of research and development services.

Income Taxes

In March 2018, the FASB issued Accounting Standards Update No. 2018-05, Income Taxes (Topic 740): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118 (“ASU 2018-05”). The standard amends ASC 740, Income Taxes (“ASC 740”), to provide guidance on accounting for the tax effects of the Tax Act pursuant to Staff Accounting Bulletin No. 118, effective immediately. The ASU permits companies to use provisional amounts for certain income tax effects of the Tax Act during a one-year measurement period. The provisional reporting period ended on December 22, 2018 and no further adjustment was required for the year ended December 31, 2018.

Stock Compensation

In May 2017, the FASB issued ASU 2017-09, Compensation – Stock Compensation (Topic 718): Scope of Modification Accounting. The new standard is intended to reduce the diversity in practice, cost and complexity when applying the guidance in Topic 718 to a change to the terms or conditions of a share-based payment award. The new standard will be effective for annual reporting periods and interim periods within those annual periods, beginning after December 15, 2018. The amendments in this update will be applied prospectively to an award modified on or after the adoption date. The Company adopted this standard as of January 1, 2018 and it did not have a material impact on the Company’s financial position or results of operations.

Recently Issued Accounting Pronouncements

In February 2016, the FASB issued ASU 2016-02 – Leases (Topic 842), which replaces the existing accounting guidance for leases. This standard requires entities that lease assets to recognize the assets and liabilities for the rights and obligations created by those leases on the balance sheet. The standard is effective for fiscal years and the interim periods within those fiscal years beginning after December 15, 2018. The guidance is required to be applied by the modified retrospective transition approach and early adoption is permitted. In July 2018, the FASB issued ASU 2018-11 Leases – Targeted Improvements, intended to ease the implementation of the new lease standard for financial statement preparers by, among other things, allowing for an additional transition method. In lieu of presenting transition requirements to comparative periods, as previously required, an entity may now elect to show a cumulative effect adjustment on the date of adoption without the requirement to recast prior period financial statements or disclosures presented in accordance with ASU 2016-02. We expect to adopt the new standard and elect to use the cumulative effect adjustment transition option effective January 1, 2019, which will be the initial date of application per ASU 2018-11.

The Company expects to elect the available package of practical expedients which allows us to not reassess previous accounting conclusions around whether arrangements are or contain leases, the classification of our leases, and the treatment of initial direct costs. The Company also expects it will make an accounting policy election to keep leases with an initial term of 12 months or less off of the balance sheet. The Company is continuing to evaluate developments within the new lease guidance and is finalizing its evaluation of its existing population of contracts to ensure all contracts that meet the definition of a lease contract under the new standard are identified. The Company has assessed the impact that the adoption of this guidance will have on its financial statements and footnote disclosures. The standard will have a material impact on the consolidated balance sheet related to the recognition of right-of-use assets and lease liabilities for operating leases. The standard will not have a material impact on the consolidated statement of operations. The Company has designed and implemented changes to related processes, controls and disclosures.

In February 2018, the FASB issued ASU 2018-02 – Income Statement – Reporting Comprehensive Income (Topic 220), which provides amended guidance on income tax accounting. The amended guidance permits the reclassification of the income tax effect on amounts recorded within other comprehensive income impacted by the Tax Cuts and Jobs Act into retained earnings. The amended guidance is effective for periods beginning after December 15, 2018 and applies only to those amounts remaining in Other Comprehensive Income at the date of enactment of the Act. The amended guidance may be adopted on either a retrospective basis or at the beginning of the period of adoption. The Company is assessing the potential impact of the amended standard but does not expect it to have a material impact on its consolidated financial statements.

In June 2018, the FASB issued ASU 2018-07 – Compensation –Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting. The standard is effective for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. Early adoption is permitted, but no earlier than an entity’s adoption date of ASC 606. The standard expands the scope of ASC 718 to include all share-based payment arrangements related to the acquisition of goods and services from both nonemployees and employees. Under the amended guidance, equity-classified share-based payment awards issued to nonemployees will be measured at grant date fair value. Upon transition, the entity is required to remeasure these nonemployee awards at fair value as of the adoption date. The Company is currently evaluating the new guidance but does not expect it to have a material impact on its consolidated financial statements.

In August 2018, the FASB issued ASU 2018-13 - Fair Value Measurement - Disclosure Framework (Topic 820). The standard modifies the disclosure requirements for fair value measurements. The standard is effective for public companies for annual and interim periods beginning after December 15, 2019. Early adoption is permitted for any removed or modified disclosures. Management is currently assessing the impact adoption will have on the Company, but it is not expected to have a material impact on the Company’s financial statement disclosures.

In August 2018, the FASB issued ASU 2018-15 - Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract. The standard requires implementation costs incurred by customers in cloud computing arrangements to be deferred over the noncancelable term of the cloud computing arrangements plus any optional renewal periods (1) that are reasonably certain to be exercised by the customer or (2) for which exercise of the renewal option is controlled by the cloud service provider. The effective date of this pronouncement is for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years, and early adoption is permitted. The standard can be adopted either using the prospective or retrospective transition approach. The Company is currently evaluating the impact of this pronouncement on the Company's consolidated financial statements and disclosures.

In November 2018, the FASB issued ASU 2018-18 - Collaborative Arrangements (Topic 808): Clarifying the Interaction Between Topic 808 and Topic 606, which, among other things, provides guidance on how to assess whether certain collaborative arrangement transactions should be accounted for under Topic 606. The amendments in this ASU are effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. The Company is in the process of evaluating the impact the standard will have on its financial statements.

Merger with Mirna Therapeutics

12 Months Ended

Dec. 31, 2018

Business Combinations [Abstract]

Merger with Mirna Therapeutics

(3)	Merger with Mirna Therapeutics

On August 28, 2017, Private Synlogic completed the Merger with Mirna as discussed in Note 1. For accounting purposes, Private Synlogic is considered to have acquired Mirna in the Merger. Private Synlogic was determined to be the accounting acquirer based upon the terms of the Merger and other factors including: (i) Private Synlogic stockholders owned approximately 83% of the combined company immediately following the closing of the Merger, (ii) Private Synlogic directors held five of the seven board seats in the combined company, and (iii) Private Synlogic management held all key positions in the management of the combined company. The Merger was accounted for as an asset acquisition rather than a business combination because the assets acquired and liabilities assumed by the Company do not meet the definition of a business as defined by ASC Topic 805, Business Combinations. The net assets acquired in connection with this transaction were recorded at their estimated acquisition date fair values as of August 28, 2017, the date the Merger was completed (the “Merger Closing Date”).

Under the terms of the Merger Agreement, Mirna issued shares of its common stock to Private Synlogic’s stockholders, at an exchange ratio of 0.5532 shares of Mirna’s common stock, after taking into account the Reverse Stock Split, for each share of Private Synlogic common stock and preferred stock outstanding immediately prior to the Merger. Mirna assumed all of the stock options outstanding under the 2017 Plan, with such stock options henceforth representing the right to purchase a number of shares of Mirna’s common stock equal to the Exchange Ratio multiplied by the number of shares of Private Synlogic common stock previously represented by such options. Mirna also assumed the 2017 Plan. The consolidated financial statements give retroactive effect to the Exchange Ratio for all periods presented.

On the Merger Closing Date, Mirna had approximately 20.9 million shares of common stock outstanding and a market capitalization of approximately $35.0 million. The estimated fair value of the net assets of Mirna on August 28, 2017 was approximately $42.6 million. The fair value of the Mirna common stock on the Merger Closing Date was below the fair value of Mirna’s net assets. As Mirna’s net assets were predominantly comprised of cash, cash equivalents and marketable securities, partially offset by current liabilities, the fair value of Mirna’s net assets as of the Merger Closing Date is considered to be the best indicator of the fair value and, therefore, the estimated preliminary purchase consideration.

All of Mirna’s assets and liabilities were reflected at their fair value on the Merger Closing Date. No goodwill or intangible assets were recognized. Consistent with accounting for an asset acquisition, the Company capitalized the costs associated with the Merger. Transaction costs primarily included bank fees and professional fees associated with legal counsel, auditors and printers. The following table shows the net assets acquired in the Merger (in thousands):

	August 28, 2017
Cash and cash equivalents	$	14,882
Marketable securities		27,600
Interest receivable		126
Prepaid assets		112
Unrealized loss on marketable securities		5
Accounts payable and accrued expenses		(105	)
Total net assets acquired		42,620
Less: Transaction costs		(2,187	)
Total net assets acquired less transaction costs	$	40,433

Fair Value of Financial Instruments

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value of Financial Instruments

(4)	Fair Value of Financial Instruments

The tables below present information about the Company’s assets that are measured at fair value on a recurring basis as of December 31, 2018 and 2017 and indicate the fair value hierarchy of the valuation techniques the Company utilized to determine such fair value, as described under Note 2, Summary of Significant Accounting Policies.

The Company’s investment portfolio includes many fixed income securities that do not always trade on a daily basis. As a result, the pricing services used by the Company applied other available information as applicable through processes such as benchmark yields, benchmarking of like securities, sector groupings and matrix pricing to prepare evaluations. In addition, model processes were used to assess interest rate impact and develop prepayment scenarios. These models take into consideration relevant credit information, perceived market movements, sector news and economic events. The inputs into these models may include benchmark yields, reported trades, broker-dealer quotes, issuer spreads and other relevant data.

At December 31, 2018 and 2017, the Company has classified assets measured at fair value on a recurring basis as follows (in thousands):

	Fair Value Measurements at Reporting Date Using
	December 31,		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
Description	2018		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	265	$	265	$	—	$	—
Corporate debt securities (included in short-term investments)		107,505		—		107,505		—
U.S. government agency securities and treasuries (included in short-term investments)		3,972		1,987		1,985		—
Total	$	111,742	$	2,252	$	109,490	$	—

	Fair Value Measurements at Reporting Date Using
	December 31,		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
Description	2017		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	21,301	$	21,301	$	—	$	—
Corporate debt securities (included in cash and cash equivalents)		11,405		—		11,405		—
Corporate debt securities (included in short-term investments)		28,585		—		28,585		—
Total	$	61,291	$	21,301	$	39,990	$	—

Cash equivalents, prepaid expenses and other current assets, accounts payable and accrued expenses at December 31, 2018 and December 31, 2017 are carried at amounts that approximate fair value due to their short-term maturities. Capital lease obligations at December 31, 2018 and December 31, 2017 approximate fair value as they bear interest at a rate approximating a market interest rate.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Available-for-Sale Investments

12 Months Ended

Dec. 31, 2018

Investments Debt And Equity Securities [Abstract]

Available-for-Sale Investments

(5)	Available-for-Sale Investments

The following tables summarize the available-for-sale securities held at December 31, 2018 and 2017 (in thousands):

December 31, 2018	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair Value
Corporate debt securities	$	107,571	$	4	$	(70	)	$	107,505
U.S. government agency securities	$	3,971	$	1	$	—		$	3,972
Total	$	111,542	$	5	$	(70	)	$	111,477

December 31, 2017	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair Value
Corporate debt securities	$	28,593	$	1	$	(9	)	$	28,585
Total	$	28,593	$	1	$	(9	)	$	28,585

The contractual maturity of all securities held at December 31, 2018 was one year or less. There were 37 investments in an unrealized loss position at December 31, 2018, none of which had been in an unrealized loss position for more than twelve months. The aggregate fair value of the securities in an unrealized loss position at December 31, 2018 and 2017 was $96.5 million and $19.3 million, respectively. The Company reviews its investments for other-than-temporary impairment whenever the fair value of an investment is less than amortized cost and evidence indicates that an investment’s carrying amount is not recoverable within a reasonable period of time. To determine whether an impairment is other-than-temporary, the Company considers whether it has the ability and intent to hold the investment until a market price recovery and considers whether evidence indicating the cost of the investment is recoverable outweighs evidence to the contrary. The Company did not hold any securities with an other-than-temporary impairment at December 31, 2018.

Gross realized gains and losses on the sales of investments have not been material to the Company’s consolidated statement of operations.

Prepaid Expenses and Other Current Assets

12 Months Ended

Dec. 31, 2018

Text Block [Abstract]

Prepaid Expenses and Other Current Assets

(6)	Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consists of the following (in thousands):

	December 31,		December 31,
	2018		2017
Prepaid insurance	$	502	$	437
Prepaid research and development		122		508
Other prepaid		597		321
Other current assets		388		298
	$	1,609	$	1,564

Property and Equipment, Net

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Property and Equipment, Net

(7)	Property and Equipment, net

Property and equipment, net consists of the following (in thousands):

	December 31,			December 31,
	2018			2017
Laboratory equipment	$	7,111		$	2,999
Computer and office equipment		781			354
Furniture and fixtures		413			220
Leasehold improvements		9,484			2,308
Construction in progress		39			7,017
		17,828			12,898
Less accumulated depreciation		(2,987	)		(3,115	)
	$	14,841		$	9,783

At December 31, 2018 and 2017, leasehold improvements include approximately $6.6 million and $1.3 million, respectively, of lessor-paid tenant improvements for which the Company was deemed to be the accounting owner of the tenant improvements primarily because it was responsible for project cost overruns. Also, at December 31, 2017, construction in progress contained approximately $5.0 million of lessor-paid tenant improvements placed in service in 2018, for which the Company was deemed to be the accounting owner primarily because it was responsible for project cost overruns.

In both 2018 and 2017, the Company entered into leases for certain laboratory equipment which were capital leases. The leases had either a present value of expected payments in excess of 90% of the fair value of the equipment or a bargain purchase option at the end of the lease. As such, as of December 31, 2018 and 2017, the Company had approximately $1.3 million and $1.4 million, respectively, of assets under a capital lease having accumulated depreciation of approximately $0.9 million and $0.2 million, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2018

Payables And Accruals [Abstract]

Accrued Expenses

(8)	Accrued Expenses

Accrued expenses consists of the following (in thousands):

	December 31,		December 31,
	2018		2017
Payroll related	$	2,906	$	1,721
Professional fees		306		805
Research and development		1,585		2,027
Other		237		270
	$	5,034	$	4,823

Common Stock

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Common Stock

(9)	Common Stock

The Company’s common stock has the following characteristics:

•

The holders of shares of common stock are entitled to one vote for each share of common stock held at all meetings of stockholders.

•

The holders of shares of common stock are entitled to receive dividends, if and when, declared by the Company’s board of directors. Since inception, no cash dividends have been declared.

The Company holds forfeiture rights relating to 118,679 shares of common stock. The forfeiture right lapses over time and is triggered when a holder ceases providing services to the Company. As of December 31, 2018, 86,581 shares of common stock have been forfeited back to the Company.

The Company holds repurchase rights which are at a price equal to the initial purchase price by the founders of Private Synlogic, adjusted by the Merger Exchange Ratio. The repurchase right lapses over time and is exercisable should the founders cease providing services to the Company prior to the end of a four-year period which began in April or May 2014, as the case may be. All repurchase rights terminated during 2018. As of December 31, 2018, the Company has exercised its repurchase right on 41,819 shares of common stock.

In January 2018, the Company sold 5,899,500 shares of its common stock through a firm commitment, underwritten public offering at a price to the public of $9.75 per share. As a result of the offering, including the exercise of the overallotment option, the Company received aggregate net proceeds, after underwriting discounts and commissions and other estimated offering expenses, of approximately $53.8 million.

In April 2018, the Company sold 3,280,000 shares of its common stock at a price of $9.15 per share in a registered direct offering. After fees and other offering expenses, the Company received approximately $28.9 million in net proceeds from the offering.

Preferred Stock

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Preferred Stock

(10)	Preferred Stock

Preferred Stock of Synlogic, Inc.

The Company’s preferred stock may be issued from time to time in one or more series, with each such series to consist of such number of shares and to have such terms as adopted by the board of directors. Authority is given to the board of directors to determine and fix such voting powers, full or limited, or no voting powers, and such designations, preferences and relative participating, optional or other special rights, and qualifications, limitation or restrictions thereof, including without limitation, dividend rights, conversion rights, redemption privileges and liquidation preferences.

Preferred Stock of Private Synlogic

Prior to the Merger, Private Synlogic had contingently redeemable preferred stock and three series of convertible preferred stock. On the Merger Closing Date, Mirna issued shares of its common stock to holders of these shares, at an exchange rate of 0.5532 shares of common stock, after taking into account the Reverse Stock Split, in exchange for each share of preferred stock outstanding immediately prior to the Merger.

Pursuant to, and at the time of, the 2017 Reorganization, preferred stock was granted to all holders of preferred units. The Synlogic preferred stock had substantially similar rights and preferences as the preferred units, except that the preferred stock was convertible into common stock at the option of the holder, on a one-for-one basis, subject to an antidilution adjustment. Conversion of the preferred stock would have been automatically triggered upon a firm-commitment underwritten public offering or upon a supermajority preferred interest vote (see (a)(v) below).

After the 2017 Reorganization, in May 2017, the Company sold and issued 2,882,679 shares of Series C preferred stock at $8.06 per share to investors for total consideration of approximately $40.4 million, net of offering costs of approximately $1.6 million. The Series C preferred stock was issued with the same terms as the then-existing preferred stock.

Rights and Preferences

Preferred stock had the following rights and preferences:

(i)

Voting

The holders of the preferred stock were entitled to vote, together with the holders of common stock, on all matters submitted to stockholders for a vote, except with respect to matters on which Delaware General Corporation Law required that a vote would be by a separate class, in which case the holders of the preferred stock would have voted separately as a class. Each holder of preferred stock was entitled to the number of votes equal to the number of shares of common stock into which each share of preferred stock was convertible at the time of such vote.

(ii)

Dividends

In the event that a dividend was declared for the holders of common stock, the holders of the preferred stock would have been entitled to the amount of dividends on an as-converted basis. Through December 31, 2018 and December 31, 2017, no dividends were declared or paid.

(iii)

Liquidation Preference

In the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company, the holders of shares of preferred stock then outstanding would have been entitled to be paid, on a pari passu basis, out of the assets of the Company available for distribution to its stockholders before any payment was made to the holders of common stock by reason of their ownership thereof, with respect to each series of preferred stock, an amount per share equal to the greater of (i) the applicable original issue price, plus any dividends declared but unpaid thereon, or (ii) such amount per share as would have been payable had all shares been converted into common stock immediately prior to such liquidation, dissolution or winding up of the Company.

If upon any such liquidation, dissolution or winding up of the Company, the assets of the Company available for distribution to its stockholders were insufficient to pay the holders of shares of preferred stock the full amount to which they should have been entitled, the holders of shares of preferred stock would share ratably in any distribution of the assets available for distribution in proportion to the respective amounts that would otherwise be payable in respect of the shares held by them upon such distribution if all amounts payable on or with respect to such shares were paid in full.

(iv)

Par Value

Par value was assigned as $0.0001.

(v)

Conversion

Each share of preferred stock, at the option of the holder, was convertible into that number of fully paid shares of common stock as determined by dividing the sum of the original issue price, plus any declared but unpaid dividends, by the conversion price in effect at the time of conversion. The initial conversion price for each share of preferred stock would have been the original issue price, subject to adjustment in accordance with antidilution provisions. Each share of preferred stock would have been automatically converted upon (i) the closing of a firm commitment underwritten public offering in which the public offering price exceeded $12.09 (adjusted to reflect subsequent stock dividends, stock splits or recapitalization) and the aggregate proceeds raised were not less than $50,000,000, or (ii) upon the vote or written consent of a supermajority preferred interest (or a majority preferred interest in the event of a public offering that did not result in the offering price or aggregate proceeds amount set forth in clause (i) above).

(vi)

Redemption

The preferred stock was not redeemable except upon a deemed liquidation event. Deemed liquidation events included a merger or acquisition in which the majority of the stock of the pre-merger corporation was not owned by the majority of the stockholders of the post-merger entity or the sale of all or substantially all of the Company’s assets. All holders of equally and more subordinated equity instruments of the Company would have been entitled to receive the same form of consideration upon the occurrence of a deemed liquidation event, consequently, the preferred stock was classified as permanent equity.

In September 2014, the Company entered into a letter agreement with the Bill & Melinda Gates Foundation (“the Gates Foundation”) with respect to the Gates Foundation purchase of 781,693 shares of the Company’s Series A Preferred Stock. The Gates Foundation investment was made in three tranches of 201,163 shares in September 2014, 218,646 shares in May 2015 and 361,884 shares in February 2016. Under the letter agreement, the Company was required to spend the approximately $5.0 million invested by the Gates Foundation for research on a particular disease, further develop the Company’s proprietary technology platform and provide assistance with access to use of such technology in developing countries. If the Company failed to spend the amount appropriately, or defaulted under certain other commitments in the agreement and the Company did not cure such default within 90 days of notice, if requested by the Gates Foundation, the Company would have been obligated to redeem the shares of Series A Preferred Stock or shares of common stock into which they had converted then held by the Gates Foundation or find a third-party to purchase such shares at a price equal to the greater of the initial purchase price and the then current fair value of such shares. In either case, if the Company, over the 6 months following such redemption, had sold substantially all of its equity or assets or completed an initial public offering at a value greater than 200% of the price paid upon redemption, then the Company would have had to reimburse the Gates Foundation for the difference. As of December 31, 2017, all obligations with respect to the Gates Foundation investment have been satisfied.

Participation Rights in Future Equity Issuances

For series of preferred stock that were issued in multiple tranches, all holders of preferred stock had a pro rata right and obligation, based on their percentage equity ownership within the series, to participate in subsequent issuances within the same series of equity securities of the Company approved by 70% vote of holders of preferred stock. Should any such holder have chosen not to purchase its full pro rata share, they would have been deemed a defaulting purchaser and all preferred stock held by a defaulting purchaser would have been automatically converted into common stock of the Company.

Preferred Units

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Preferred Units

(11)	Preferred Units

Prior to the 2017 Reorganization, the Company had one class of contingently redeemable preferred units and two classes of convertible preferred units. Pursuant to the 2015 Reorganization, each share of the Company’s Series A Preferred Stock and Series A Contingently Redeemable Preferred Stock was exchanged for a like type and number of the Company’s Class A Preferred Units and Contingently Redeemable Class A Preferred Units, respectively.

In February 2016, Synlogic issued and sold 2,005,348 units of Class A-3 Preferred Units and 361,884 units of Contingently Redeemable Class A-3 Preferred Stock at $7.23 per unit to investors for net proceeds of approximately $17.1 million. There were no issuance costs related to these transactions.

In February 2016, Synlogic also issued and sold 1,029,850 units of Class B Preferred Units at $13.53 per unit to investors for net proceeds of approximately $13.6 million. Issuance costs related to this transaction of approximately $0.3 million were recorded as a reduction of proceeds within Class B Preferred Units (together with the Class A Preferred Units, Contingently Redeemable Class A Preferred Units, Class A-2 Preferred Units, Class A-2 Contingently Redeemable Preferred Units, Class A-3 Preferred Units and Contingently Redeemable Class A-3 Preferred Units, the “Preferred Units”).

Rights and Preferences

The Preferred Units had substantially similar rights and preferences as were conferred upon the preferred stock as follows:

(i)

Voting

The holders of the Preferred Units were entitled to vote, together with the holders of the Company’s common units as a single class, on all matters submitted to unit holders for a vote. In addition, holders of at least a majority of the outstanding Preferred Units and common units voting as a single class were entitled to take any action required or permitted to be taken at any meeting of the members, unless a different vote is required by the Delaware Limited Liability Company Act or the Company’s operating agreement.

(ii)

Distributions

Distributions were governed by the Company’s operating agreement (Note 13). No distributions were made in either of the years ended December 31, 2018 or December 31,2017.

(iii)

Liquidation Preference

In the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company, the assets of the Company would have been distributed, after the payout or provision for payment of all creditors of the Company, in accordance with the same order of priority as distributions (Note 13).

(iv)

Par Value

The Preferred Units did not have a par value.

(v)

Redemption

The Preferred Units were not redeemable except upon a deemed liquidation event. Deemed liquidation events included the merger, acquisition or sale of all or substantially all of the Company’s assets. All holders of equally and more subordinated equity instruments of the Company would have been entitled to receive the same form of consideration upon the occurrence of a deemed liquidation event, consequently, the Preferred Units were classified as permanent equity.

In September 2014, the Company entered into a letter agreement with the Bill & Melinda Gates Foundation (“the Gates Foundation”) (Note 10) with respect to the Gates Foundation purchase of 781,693 shares of the Company’s Series A Preferred Stock. The Gates Foundation investment was made in three tranches of 201,163 shares in September 2014, 218,646 shares in May 2015 and 361,884 units in February 2016. The first two tranches, totaling 419,809 shares were exchanged for Class A Preferred Units pursuant to the 2015 Reorganization in July 2015. As a result, 781,693 units of Class A Preferred Units with a cost of approximately $5.0 million were classified as Contingently Redeemable Preferred Units in mezzanine equity, as of December 31, 2016.

(vi)

Participation Rights

Holders of Class A Preferred Units had the right and obligation to participate in additional closings of Class A Preferred Units upon the achievement of certain milestones by the Company. If any holder of Class A Preferred Units did not purchase the number of Class A Preferred Units required to be purchased by it at any such additional closing, then each Class A Preferred Unit held by such member would have automatically been converted into common units at the applicable adjustment ratio in effect with respect to such units immediately prior to such closing. All holders of Class A Preferred Units participated in additional closings at the required levels. Holders of Class B Preferred Units had the right and obligation to participate in additional closings of Class B Preferred Units upon the achievement of certain milestones by the Company. If any holder of Class B Preferred Units did not purchase the number of Class B Preferred Units required to be purchased by it at any such additional closing, then each Class B Preferred Unit held by such member would have automatically been converted into common units at the applicable adjustment ratio in effect with respect to such units immediately prior to such closing.

(vii)

Initial Public Offering

In connection with preparation for an initial public offering, upon request of holder of at least 70% of the Preferred Units, all unit holders would have been required to have taken appropriate steps to implement a reorganization of the Company that may have included, for example, contribution of their units to a newly formed corporation.

Equity-based Compensation and Equity Incentive Plans

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Equity-based Compensation and Equity Incentive Plans

(12)	Equity‑based Compensation and Equity Incentive Plans

Equity Plans

The Company has a number of equity plans, two of which are currently active.

The 2015 Equity Incentive Award Plan (“2015 Plan”) was adopted by Mirna in 2015 and remains active after the Merger, now functioning as the primary equity plan for the Company. Following the Merger, there were 647,893 shares authorized under the 2015 Plan. The 2015 Plan includes an “evergreen provision” that allows for an annual increase in the number of shares of common stock available for issuance under the 2015 Plan, which annual increase will be added on the first day of each fiscal year from 2016 through 2025, inclusive, and will be equal to the lesser of (i) five percent of the shares outstanding on the last day of the immediately preceding fiscal year and (ii) such smaller number of shares as determined by the Board of Directors. The 2015 Plan provides for the granting of a variety of stock‑based compensation awards, including stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, deferred stock awards, dividend equivalent awards, stock payment awards, performance awards and other stock‑based awards.

The 2017 Stock Incentive Plan was adopted by Private Synlogic in 2017 at the time of the 2017 Reorganization and provides for the grant of incentive stock options, non-qualified stock options, restricted and unrestricted stock awards and other stock-based awards. Under the 2017 Plan, 1,753,061 shares were initially authorized and reserved for issuance. Pursuant to the 2017 Reorganization, Private Synlogic issued restricted common stock awards under the 2017 Plan to replace the canceled incentive units pursuant to the termination of the 2015 LLC Plan (“2015 LLC Plan”). In addition, Private Synlogic also issued stock options to certain employees prior to the Merger. Pursuant to the Merger Agreement, each restricted common stock award of Private Synlogic under the 2017 Plan that was outstanding immediately prior to the Merger and each option to purchase common stock of Private Synlogic under the 2017 Plan that was outstanding and unexercised immediately prior to the Merger was converted into and became restricted common stock and options to purchase shares of the Company’s common stock, respectively, based on the Exchange Ratio of 0.5532 and the Company assumed the 2017 Plan.

The 2015 Employee Stock Purchase Plan (“ESPP”) was adopted by Mirna in 2015 and allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 15% of their eligible compensation, subject to any plan limitations. The ESPP generally provides for set offering periods, and at the end of each offering period, employees are able to purchase shares at 85% of the lower of the fair market value of the Company’s common stock on the first trading day of the offering period or on the last trading day of the offering period. The Company suspended the ESPP in 2017.

The 2008 Long Term Incentive Plan (“2008 Plan”) was adopted by Mirna in 2008 and allowed for the grant of incentive stock options to employees and nonqualified stock options and other equity awards to employees and nonemployees. The 2015 Plan is the successor to the 2008 Plan and at the time of the Merger, the remaining awards outstanding thereunder were cancelled and the number of shares with respect to those awards were transferred to the 2015 Plan. As of the Merger, the 2008 Plan was retired.

The 2015 LLC Plan was adopted by Private Synlogic at the time of the 2015 Reorganization, which provided for the grant of equity incentive units to employees, officers, directors or consultants. The 2015 LLC Plan was cancelled pursuant to the 2017 Reorganization as described above.

As of December 31, 2018, there were 477,414 shares available for future grant under the Company’s two active equity incentive plans, the 2017 Plan and the 2015 Plan.

The Company is displaying all equity in its post-Merger amounts, as impacted by the Exchange Ratio.

Stock Options

The weighted average assumptions used in the Black-Scholes option-pricing model for stock options issued to employees under its two active equity plans, the 2015 Plan and the 2017 Plan, during the years ended December 31, 2018 and 2017 and to nonemployees during the year ended December 31, 2017 were:

	Year ended December 31,
Employees:	2018			2017
Expected term	6.2 years			6.2 years
Weighted-average, risk-free interest rate		2.8	%		2.1	%
Expected volatility		70.8	%		70.4	%
Dividend yield		—			—

The following table summarizes stock option activity, as adjusted for the Exchange Ratio under the 2015 and 2017 Plans.

	Stock options outstanding
						Weighted
						average
				Weighted		remaining		Aggregate
				average		contractual		intrinsic
	Number of			exercise		term		value^(a)
	options			price		(in years)		(in thousands)
Outstanding at December 31, 2017		1,267,221		$	13.62		9.6	$	—
Granted		919,496			9.71
Exercised		(19,830	)		12.28
Forfeited		(394,136	)		12.09
Expired		(32,867	)		13.53
Outstanding at December 31, 2018		1,739,884			11.92		9.0	$	—

Vested or expected to vest at December 31, 2018		1,739,884			11.92		9.0	$	—
Exercisable at December 31, 2018		489,236			13.20		8.4	$	—

^(a)	The aggregate intrinsic value is calculated as the difference between the exercise price of the options and the fair market value of the underlying common stock for the options that were in the money at December 31, 2018 . No options were in the money at December 31, 2018 .

During the years ended December 31, 2018 and 2017, 919,496 and 1,281,647 stock options were granted to employees and nonemployees, respectively. Approximately $3.2 million and $2.0 million in equity compensation was recognized related to stock options granted to employees and nonemployees for the years ended December 31, 2018 and 2017, respectively.

The weighted average grant date fair value per share of options granted to employees during the year ended December 31, 2018 was approximately $6.35. The total fair value of awards that vested during the year ended December 31, 2018 was $3.3 million.

As of December 31, 2018, there was approximately $8.4 million of unrecognized share-based compensation related to employees for unvested stock option grants which is expected to be recognized over a weighted average period of 2.6 years. The total unrecognized share-based compensation cost will be adjusted for actual forfeitures as they occur. In addition, there was approximately $53,000 of unrecognized share-based compensation, related to unvested stock option grants to nonemployees which is expected to be recognized over a weighted average period of 2.4 years. The amount of equity-based compensation expense related to nonemployees that will ultimately be recorded will depend on the remeasurement of the outstanding awards through their vesting date.

Restricted Common Stock

During the year ended December 31, 2018, no shares of restricted common stock were granted. During the year ended December 31, 2017, 1,062,794 shares of restricted common stock were granted, including 1,059,912 shares of restricted common stock (adjusted for the Exchange Ratio) granted in exchange for the restricted common units and incentive units that were cancelled as part of the 2017 Reorganization. These shares retained the same vesting schedule as the cancelled restricted common units and incentive units. Private Synlogic treated these as modifications to the original grants of incentive units because the cancellation and reissuance was deemed to be concurrent. The calculation of the incremental compensation expense was based on the excess of the fair value of the award measured immediately before and after the modification. As a result of the modification, Private Synlogic recognized approximately $26,000 in equity-based compensation.

The following table shows restricted common stock activity:

	Restricted stock awards
				Weighted average grant date
	Number of			fair value
	shares			(per share)
Unvested at December 31, 2016		—		$	—
Awards exchanged upon 2017 Reorganization		1,059,912			13.53
Granted		2,884			19.01
Vested		(671,204	)		13.54
Forfeited		(16,113	)		13.53
Unvested at December 31, 2017		375,479		$	13.55
Granted		—			—
Vested		(186,332	)		13.54
Forfeited		(70,468	)		13.61
Unvested at December 31, 2018		118,679		$	13.54

During the years ended December 31, 2018 and 2017, 186,332 and 671,204 shares of restricted common stock vested and approximately $1.0 and $0.5 million in equity compensation was recognized, respectively.

As of December 31, 2018, there was approximately $0.2 million of unrecognized share-based compensation related to restricted stock awards granted to employees, which is expected to be recognized over a weighted average period of 1.6 years. The total unrecognized share-based compensation cost will be adjusted for actual forfeitures as they occur. There was no unrecognized share-based compensation related to unvested restricted stock awards granted to nonemployees at December 31, 2018.

Incentive Units

Incentive units issued by Synlogic, LLC under the 2015 LLC Plan generally vested 25% after one year and ratably monthly thereafter over the next 36 months. Certain awards provided for accelerated vesting upon a change in control, as defined in the 2015 LLC Plan. Incentive units did not expire. Holders of incentive units had no voting rights in connection with such incentive units. Each incentive unit was intended to be a profits interest within the meaning of IRS regulations promulgated under the Internal Revenue Code. Each incentive unit had a threshold price, which was the price above which an incentive unit would participate in distributions. In this way, an incentive unit was designed to participate in the future profits and appreciation of Synlogic, LLC. Holders of incentive units would have been entitled to receive profits when and if distributions were in excess of the threshold price of the award set by the Board of Directors on the date of grant.

Synlogic, LLC measured and recorded the value of incentive units granted to nonemployees over the period of time that services were provided and, as such, unvested portions were subject to remeasurement at subsequent reporting periods.

No incentive units were issued during the years ended December 31, 2018 and 2017. In May 2017, all incentive units were cancelled pursuant to the 2017 Reorganization and reissued as restricted common stock. As a result, there was no incentive unit activity during the year ended December 31, 2018 or unrecognized compensation expense related to incentive units as of December 31, 2018

The following table represents a summary of incentive unit activity, as adjusted for the Merger, under the 2015 LLC Plan:

	Incentive units
				Weighted-		Weighted-		Weighted-
				average		average		average
	Number of			strike		threshold		grant date
	units			price		price		fair value
Non-vested units at December 31, 2016		971,906		$	5.22	$	5.93	$	1.01
Granted		—			—		—		—
Vested		(73,719	)		4.01		5.53		0.87
Forfeited		(260,145	)		4.19		5.57		1.05
Non-vested units cancelled upon 2017 Reorganization		(638,042	)		5.78		6.15		1.05
Non-vested units at December 31, 2017		—		$	—	$	—	$	—

Vested or expected to vest at December 31, 2017		—		$	—	$	—	$	—

Restricted Common Units

In May 2017, all restricted common unit awards were cancelled pursuant to the 2017 Reorganization and reissued as restricted common stock. As a result, there was no unrecognized compensation expense related to unvested restricted common units as of December 31, 2018.

The following table shows the restricted common unit activity for the year ended December 31, 2017, prior to the 2017 Reorganization, as adjusted for the Merger:

	Restricted common units
				Grant date
	Number of			fair value
	units			(per unit)
Unvested at December 31, 2016		219,087		$	1.48
Granted		—			—
Vested		(37,770	)		1.48
Forfeited		—			—
Exchanged as part of 2017 Reorganization		(181,317	)		1.48
Unvested at December 31, 2017		—		$	—

Equity Compensation

The Company has recorded total equity‑based compensation expense of approximately $4.3 million and $2.7 million, during the years ended December 31, 2018 and 2017, respectively. Equity compensation during the years ended December 31, 2018 and 2017 is derived from stock options and restricted stock awards. Equity-based compensation during the year ended December 31, 2018 also includes $0.7 million related to modifications in equity awards in connection with the separation of the Company’s former Chief Executive Officer. Equity compensation during the year ended December 31, 2017 additionally includes equity compensation derived from incentive units and restricted common units granted prior to the 2017 Reorganization.

In July 2015, in connection with the 2015 Reorganization, all outstanding stock options and awards were canceled and reissued as incentive units and restricted common units. As such, equity compensation prior to the Merger was derived from incentive units and from a grant of restricted common units.

In May 2017, in connection with the 2017 Reorganization, the incentive units and restricted common units were cancelled and exchanged for restricted stock awards and stock options. Equity compensation after May 2017, was derived from stock options and restricted stock awards.

The following table summarizes equity‑based compensation expense within the Company’s consolidated statements of operations and comprehensive loss for the years ended December 31, 2018 and 2017 (in thousands):

	Years ended December 31,
	2018		2017
Research and development	$	1,333	$	1,410
General and administrative		2,984		1,242
	$	4,317	$	2,652

The following table summarizes equity‑based compensation expense by type of award for the years ended December 31, 2018 and 2017 (in thousands):

	Years ended December 31,
	2018		2017
Stock options	$	3,361	$	1,956
Restricted stock awards		956		508
Incentive units		—		132
Restricted common units		—		56
	$	4,317	$	2,652

Distributions

12 Months Ended

Dec. 31, 2018

Distributions [Abstract]

Distributions

(13)	Distributions

The Board of Directors of Synlogic, LLC had the authority to determine the amount, if any, of proceeds available for distribution to unit holders. In the event that a distribution of proceeds was declared by the Board of Directors, such proceeds would have been distributed in accordance with the following order of priority:

•

first, to holders of Class B Preferred Units, pro rata in proportion to their unpaid contributed capital, until such holder had received an amount equal to its capital contribution;

•

second, to holders of Class A Preferred Units and Class A Contingently Redeemable Preferred Units, pro rata in proportion to their unpaid contributed capital, until such holder had received an amount equal to its capital contribution;

•

third, to all holders of preferred units, common units and incentive units, pro rata in proportion to the remaining amount to be distributed, until an aggregate amount had been distributed in respect of each preferred unit, common unit and incentive unit equal to the greatest aggregate amount per unit distributed in respect of any preferred unit under the first and second priority described above; provided, that no holder of an incentive unit shall participate in any distributions until a total amount equal to the threshold price with respect to such incentive unit has been distributed in respect of any common unit outstanding on the date of issuance of such incentive unit subsequent to the issuance of such incentive unit;

•

fourth, to each holder of certain incentive units for which the Board of Directors had established a strike price, pro rata in proportion to the remaining amount to be distributed, an amount equal to the difference between the strike price for such incentive unit, and the threshold price for such incentive unit; and

•

thereafter, to all holders of preferred units, common units and incentive units, pro rata in proportion to their percentage interest.

No distributions were made to unit holders prior to the 2017 Reorganization.

Significant Agreements

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Significant Agreements

(14)	Significant Agreements

AbbVie Collaboration Agreement

In July 2015, the Company entered into the AbbVie Agreement under which the Company granted AbbVie an exclusive option to purchase IBDCo and, in exchange, agreed to collaborate in researching and developing an Investigational New Drug (“IND”) candidate for the treatment of IBD. The AbbVie Agreement sets forth the Company’s and AbbVie’s respective obligations for development and delivery of an IND candidate package using reasonable commercial efforts.

In exchange for the exclusive option to acquire IBDCo, initial research and development services for drug discovery and pre-clinical development, and participation on the joint research committee (“JRC”), AbbVie agreed to pay IBDCo an upfront, nonrefundable cash payment of $2.0 million, which IBDCo received in December 2015. AbbVie also agreed to pay IBDCo up to $16.5 million in milestone payments associated with specified research and pre-clinical events, which were determined to represent customer options for accounting purposes, as well as an option exercise fee upon the execution of their option to buy IBDCo and other royalty and milestone payments. The upfront cash payment and any payments for option fees and royalties are non-refundable, non-creditable and not subject to set-off.

The research and development will be performed by the Company over four phases of research defined in the research plan. The Company is eligible to receive payments from AbbVie upon the election to continue the research and development at the achievement of certain milestone events. The JRC will make a determination as to the continuation of the collaboration at the achievement of research and pre-clinical milestones, except for the final milestone, which AbbVie has the discretion to determine achievement without the approval of the JRC. If the parties make the determination to continue on with the AbbVie Agreement upon achievement of each milestone event, then AbbVie will pay the consideration associated with that milestone and the collaboration will continue through the remaining term of the option to purchase IBDCo, which was initially considered to be approximately 54 months. However, AbbVie has the right to terminate the contract at any time with 90 days’ notice.

The Company assessed this arrangement in accordance with ASC 606 and concluded that the contract counterparty, AbbVie, is a customer. The Company identified the following material promises at the outset of the arrangement: (1) a non-exclusive royalty-free research and development license; (2) research and development services for pre-clinical activities under the research plan through to the first research and development phase (or an estimated 17 months); (3) three option rights for AbbVie to continue the collaboration as related to three phases of research and development; (4) participation on the JRC; and (5) the transfer of ownership of IBDCo upon exercise of the option to buy IBDCo. The Company determined that the license and research and development activities were not distinct from one another. Participation on the JRC to oversee the research and development activities was determined to be quantitatively and qualitatively immaterial and therefore is excluded from performance obligations. As such, the Company determined that the license and research and development services should be combined into a single performance obligation.

The Company evaluated the milestone payments, which represent customer options as described above, and the option to purchase IBDCo, to determine whether they provide AbbVie with any material rights. The Company concluded that the options were not issued at a significant and incremental discount, and therefore do not provide material rights. As such, they were excluded as performance obligations at the outset of the arrangement. If AbbVie elects to exercise the options, the additional consideration will be added to the transaction price and allocated to the resulting performance obligations.

Based on these assessments, the Company identified one performance obligation at the outset of the AbbVie Agreement, which consists of: (1) the non-exclusive license and (2) the research and development activities through the first research and development phase.

At the outset of the arrangement, the transaction price included only the $2.0 million up-front consideration received which was allocated to the single performance obligation. The option exercise fees ($16.5 million for the milestones and the IBDCo purchase option exercise fee) that may be received are excluded from the transaction price until each customer option is exercised. The Company reevaluates the transaction price at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur, and, if necessary, adjust its estimate of the transaction price.

In May 2017, the Company completed the research and development services for the first phase of the research plan and was paid $2.0 million to commence the second phase of the research plan. At this time, the $2.0 million was added to the transaction price and allocated to a new performance obligation consisting of the underlying license and research and development services to be performed over the second phase of the research plan.

On September 27, 2018, AbbVie and the Company signed an amendment (the “Amendment”) to the AbbVie Agreement. The Amendment clarified the requirements necessary to complete the second phase which resulted in additional time and effort in the second phase of the research plan. Additionally, the Amendment split the next milestone payment under the AbbVie Agreement into two payments: a milestone payment of $2.0 million earned by the Company upon execution of the Amendment and the remaining milestone payment of the balance due upon the successful achievement of specified research and pre-clinical events and the advancement to the third phase of the research plan.

The Company determined that the Amendment represented a modification to the AbbVie Agreement. The additional research and development services are not distinct from the remaining research and development services under the second phase of the research plan of the AbbVie Agreement. The Amendment was accounted for as part of the original AbbVie Agreement and the services form part of the single performance obligation that was partially satisfied as of the date of the contract modification. As a result, the transaction price for the current performance obligation associated with the second phase of the research plan increased by $2.0 million. The impact of the contract modification on the transaction price and the measure of progress toward completion of the performance obligation was recognized as an adjustment to revenue upon execution of the Amendment on a cumulative catch-up basis. The cumulative catch-up adjustment to revenue, as a result of the contract modification, was $1.8 million recognized during September 2018. Additionally, deferred revenue increased by $0.3 million as a result of the contract modification.

Revenue associated with performance obligations under the AbbVie Agreement are recognized as the research and development services are provided using an input method, according to the full time equivalents incurred. The research and development activities are expected to be performed over a period of approximately 54 months. The transfer of control occurs over time and, in management’s judgment, is the best measure of progress towards satisfying the performance obligation. The amounts received that have not yet been recognized as revenue are recorded in deferred revenue on the Company’s consolidated balance sheet.

For the years ended December 31, 2018 and 2017, the Company had recognized $ 2.5 million and $2.4 million, respectively, as collaboration revenue in the Company’s consolidated statements of operations and comprehensive loss. Deferred revenue amounted to $0.3 million as of December 31, 2018, all of which is included in current liabilities.

License Agreement with the Massachusetts Institute of Technology and Boston University

In April 2017, the Company exercised an option associated with the October 2014 agreement with Boston University and the Massachusetts Institute of Technology to acquire a license for certain intellectual property in exchange for $50,000. The execution of this option triggered an equity award for the issuance of 325,377 common units, which were converted to 325,377 common shares upon the 2017 Reorganization and converted to 179,999 common shares during the Merger. Based on the fair value of common units at the time of the execution of the license, the Company recognized license fees of approximately $1.8 million upon issuance of the common units associated with the equity award. Additionally, the Company was required to pay approximately $0.3 million for prior patent costs incurred in connection with the option agreement. The Company recorded these amounts, including the fair value of the common stock issued to the licensors as research and development expense in the 2017 consolidated statement of operations, as the licenses do not have future alternative use, in accordance with ASC Topic 730, Research and Development.

The Company determined that its growing portfolio of internally generated intellectual property superseded the in-licensed intellectual property in the BU license and the MIT license. Accordingly, on December 18, 2018, it provided notice to terminate the license agreements with MIT and BU/MIT. The BU license will be terminated effective as of February 16, 2019 and the MIT license will be terminated effective as of June 19, 2019.

Sales Agreement with Cowen and Company

On October 13, 2017 the Company entered into a sales agreement with Cowen and Company, LLC (“Cowen”) with respect to an at-the-market (“ATM”) offering program under which the Company may offer and sell, from time to time at its sole discretion, shares of its common stock through Cowen as its sales agent. In an ATM offering, exchange-listed companies incrementally sell newly issued shares into the secondary trading market through a designated broker-dealer at prevailing market prices. No sales of common stock were made pursuant to the ATM during 2017 and 2018.

Net Loss per Share

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Net Loss per Share

(15)	Net Loss per Share

The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders (in thousands, except for share and per share amounts):

	2018			2017
Numerator:
Net loss attributable to common stockholders	$	(48,435	)	$	(40,377	)
Denominator:
Weighted-average common shares outstanding - basic and diluted		23,882,685			6,724,641
Net loss per share attributable to common stockholders - basic and diluted	$	(2.03	)	$	(6.00	)

The Company’s potentially dilutive shares, which include outstanding stock options and unvested restricted common stock, are considered to be common share equivalents and are only included in the calculation of diluted net loss per share when their effect is dilutive.

The following potential common shares, presented based on amounts outstanding at each period end, were excluded from the calculation of the diluted net loss per share attributable to common stockholders for the period indicated because including them would have had an anti-dilutive effect.

	As of December 31,
	2018		2017
Unvested restricted common stock awards		118,679		375,479
Outstanding options to purchase common stock		1,739,884		1,267,221

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

(16)	Income Taxes

The provision for income taxes consist of the following (in thousands):

	December 31,
	2018		2017
Current Tax Expense:
Federal	$	—	$	—
State		33		—
	$	33	$	—

Deferred taxes are recognized for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes. Deferred tax assets consist of the following (in thousands):

	December 31,
	2018			2017
Deferred tax assets:
Net operating loss carryforwards	$	33,275		$	21,248
Tax credit carryforwards		4,365			3,038
Accrued expenses		103			32
Property and equipment		—			390
Deferred rent		2,209			53
Equity compensation		784			503
Amortizable intangibles		1,339			1,492
Other		78			—
Gross deferred tax assets		42,153			26,756
Deferred tax liability:
Other		—			(241	)
Property and equipment		(1,863	)		—
Valuation allowance		(40,290	)		(26,515	)
Net deferred tax assets	$	—		$	—

Management of the Company has evaluated the positive and negative evidence bearing upon the realizability of the Company’s deferred tax assets, which are comprised principally of net operating loss carryforwards, and determined that it is more likely than not that the Company will not recognize the benefits of the deferred tax assets. As a result, a full valuation allowance of approximately $40.3 million and $26.5 million was established at December 31, 2018 and 2017, respectively.

A reconciliation of the statutory federal income tax rate to the Company’s effective income tax rate is as follows (dollars in thousands):

	Years ended December 31,
	2018			2017
	Tax Rate			Tax Rate
U.S. federal statutory rate		21	%		34	%
State income taxes, net of federal benefit		6	%		5	%
Other permanent differences		(1	)%		(1	)%
Tax credits		3	%		5	%
Other items		(1	)%		—
Change in rate due to Tax Reform		—			(27	)%
Mirna acquisition		—			17	%
Net change in valuation allowance		(28	)%		(33	)%
Effective income tax rate		—			—

A roll-forward of the valuation allowance for the years ended December 31, 2018 and 2017 is as follows (in thousands):

	Years ended December 31,
	2018			2017
Balance at beginning of year	$	(26,515	)	$	(13,060	)
Increase in valuation allowance		(13,775	)		(13,455	)
Balance at end of year	$	(40,290	)	$	(26,515	)

As of December 31, 2018 and 2017, the Company had federal and state net operating loss carryforwards that may be available to reduce future taxable income of approximately $125.5 million and $82.0 million, respectively, which begin to expire in 2034. In addition, at December 31, 2018, the Company had federal and state research and development tax credit carryforwards available to reduce future tax liabilities of approximately $2.8 million and $1.6 million, respectively. These credits begin to expire in 2034 and 2029, respectively.

Pursuant to Section 382 of the Internal Revenue Code of 1986 (“IRC”), certain substantial changes in the Company’s ownership may result in a limitation on the amount of net operating loss (“NOL”) carryforwards and research and development credit (“R&D credit”) carryforwards that may be used in future years. Utilization of the NOL and R&D credit carryforwards may be subject to a substantial annual limitation under Section 382 of the IRC due to ownership change limitations that have occurred previously or that could occur in the future. These ownership changes may limit the amount of NOL and R&D credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. The Company has not completed a study to assess whether an ownership change has occurred, or whether there have been multiple ownership changes since its formation, due to a significant complexity and related costs associated with such a study. There could be additional ownership changes in the future that may result in additional limitations on the utilization of NOL carryforwards and credits.

The Company is required to determine whether a tax position of the Company is more likely than not to be sustained upon examination, including resolution of any related appeals of litigation processes, based on the technical merits of the position. For tax positions meeting the more likely than not threshold, the tax amount recognized in the financial statements is reduced by the largest benefit that has a greater than fifty percent likelihood of being realized upon the ultimate settlement with the relevant taxing authority. The Company has not recognized any liability for unrecognized tax benefits as of December 31, 2018.

The Company files tax returns, on an entity-level basis, as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. Tax years from 2015 to the present are open to examination under the statute. The Company’s policy is to record interest and penalties related to income taxes as part of the tax provision. There are no interest or penalties accrued at December 31, 2018 and 2017.

Effects of the Tax Cuts and Jobs Act

On December 22, 2017, President Trump signed into U.S. law the Tax Cuts and Jobs Act of 2017 (“Tax Reform”). ASC Topic 740, Accounting for Income Taxes, requires companies to recognize the effect of tax law changes in the period of enactment even though the effective date for most provisions is for tax years beginning after December 31, 2017, or in the case of certain other provisions of the law, January 1, 2018. Given the significance of the legislation, the U.S. Securities and Exchange Commission (the "SEC") staff issued Staff Accounting Bulletin ("SAB") No. 118 (“SAB 118”), which allows registrants to record provisional amounts during a one year “measurement period” similar to that used when accounting for business combinations. However, the measurement period is deemed to have ended earlier when the registrant has obtained, prepared, and analyzed the information necessary to finalize its accounting. During the measurement period, impacts of the law are expected to be recorded at the time a reasonable estimate for all or a portion of the effects can be made, and provisional amounts can be recognized and adjusted as information becomes available, prepared, or analyzed.

SAB 118 summarizes a three-step process to be applied at each reporting period to account for and qualitatively disclose: (1) the effects of the change in tax law for which accounting is complete; (2) provisional amounts (or adjustments to provisional amounts) for the effects of the tax law where accounting is not complete, but that a reasonable estimate has been determined; and (3) a reasonable estimate cannot yet be made and therefore taxes are reflected in accordance with law prior to the enactment of the Tax Cuts and Jobs Act.

However, several provisions of the Tax Reform have significant impact on the Company’s U.S. tax attributes, generally consisting of credits, loss carry-forwards, and amortizable intangibles. The provisional reporting period ended on December 22, 2018. The Company has reevaluated its assets and liabilities associated with such future tax benefits in the current year and determined that no further adjustment is necessary to its deferred tax asset and liabilities. This reduction in the deferred tax asset has been offset by a coinciding reduction in the associated valuation allowance, creating a zero net impact to the Company’s statement of operations. The Company’s tax attributes are generally subject to a full valuation allowance in the United States and thus, any adjustments to the attributes will not impact the tax provision. Although the Company has made a reasonable estimate of the gross amounts of the attributes disclosed, a final determination of the Tax Reform’s impact on the attributes and related valuation allowance requirements remain incomplete pending a full analysis of the provisions and their interpretations.

Other significant provisions that are not yet effective but may impact income taxes in future years include: a limitation on the current deductibility of net interest expense in excess of 30 percent of adjusted taxable income and a limitation of net operating losses generated after fiscal 2018 to 80 percent of taxable income.

Leases

12 Months Ended

Dec. 31, 2018

Leases [Abstract]

Leases

(17)

Leases

The Company recorded rent expense of approximately $2.5 million for the year ended December 31, 2018. The Company recorded a rent credit of approximately $0.2 million for the year ended December 31, 2017 due to the accelerated amortization the deferred rent associated with the 200 Sidney Street facility. In July 2017, the Company agreed to terminate its lease and revised its estimate of the remaining amortization period from 63 months to seven months.

Operating Leases

In July 2017, the Company entered into an agreement to lease approximately 41,346 square feet of laboratory and office space at 301 Binney Street in Cambridge, Massachusetts. Annual rent is approximately $3.1 million. The ten-year lease commenced in January 2018 and contains provisions for a free-rent period, annual rent increases and an allowance for tenant improvements. The Company is responsible for real estate taxes, maintenance, and other operating expenses applicable to the leased premises. In addition to approximately $1.6 million the Company has committed to for tenant improvements, the operating lease also provided for a tenant improvement allowance, at the cost of the lessor, not to exceed approximately $6.6 million. The Company was deemed to be the accounting owner of the tenant improvements primarily because it was responsible for project cost overruns. Therefore, the amounts will be recorded as a leasehold improvement and deferred rent and will be recorded as a reduction to rent expense ratably over the lease term. At December 31, 2018, the Company has capitalized approximately $5.0 million of the landlord-funded tenant improvements, representing the completed portion of the buildout. In conjunction with the lease, the Company established a letter of credit of approximately $1.0 million secured by cash balances included in restricted cash.

In July 2015, the Company entered into an operating lease for office and laboratory space at 200 Sidney Street in Cambridge, Massachusetts. The operating lease term commenced in February 2016 and expired in April 2021 with a one year renewal option to extend the lease. The Company agreed to terminate the lease in July 2017 at a date that was 30 days after the commencement of its new lease. No penalties were associated with the termination of the lease. Rent expense commenced on February 1, 2016 and was recognized on a straight‑line basis over the duration of the term. The operating lease provided for annual rent of approximately $0.9 million, payable on a monthly basis, which increased at a rate of 3% annually, and included three months of rent abatement during the first year. The Company was responsible for real estate taxes, maintenance, and other operating expenses applicable to the leased premises. The Company was deemed to be the accounting owner of the tenant improvements primarily because it was responsible for project cost overruns. Therefore, the amounts were recorded as a leasehold improvement and deferred rent and were being recorded as a reduction to rent expense ratably over the lease term of 63 months. As a result of the agreement to terminate its lease, the Company revised its estimate of the remaining amortization period of the deferred rent and its estimate of the remaining useful life of its leasehold improvements to seven months through January 2018.

Capital Leases

In June 2017, the Company entered two non-cancellable thirty-six month lease agreements for certain lab equipment of approximately $0.2 million and $0.7 million, respectively. The lease term and payments for each agreement began upon delivery and installation of the equipment. Both leases are accounted for as a capital lease as one has a bargain purchase option and in the other, the present value of the lease exceeds 90% of the fair market value. At December 31, 2018, the interest rate on each capital lease obligation was approximately 1.1% and 7.3%, respectively.

In October 2016, the Company entered into a twenty-four month, non-cancellable lease agreement for approximately $0.4 million for certain lab equipment. Due to the existence of a bargain purchase option, the lease has been accounted for as a capital lease. At December 31, 2017, the interest rate on the outstanding capital lease obligation was approximately 9.6%. The agreement terminated per the terms of the lease agreement in November 2018.

Future minimum lease payments under the Company’s non-cancelable operating and capital leases as of December 31, 2018, are as follows (in thousands):

	Operating		Capital
	leases		leases
Fiscal year:
2019	$	3,175	$	287
2020		3,270		214
2021		3,369		2
2022		3,470		—
2023		3,574		—
Thereafter		18,067		—
Total future minimum lease payments	$	34,925	$	503
Less amounts representing interest				27
Capital lease obligations at December 31, 2018				476
Less current portion of capital lease obligations				266
Capital lease obligations, net of current portion			$	210

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

(18)	Commitments and Contingencies

On December 7, 2018, Synlogic Operating Company, Inc., a wholly-owned subsidiary of Synlogic, Inc. (the “Company”), entered into a Statement of Work (the “SOW”) with Azzur Group, LLC (“Azzur”) pursuant to a Master Contract Services Agreement (the “Master Services Agreement”), dated September 8, 2018, between the Company and Azzur.

Pursuant to the SOW, Azzur has agreed to provide the Company with access to, and the use of, an approximately 700 square foot cleanroom space to be constructed in Waltham, Massachusetts (the “Azzur Suite”), for a period of 44 months, from May 1, 2019 to December 31, 2022 (the “Term”). Azzur has also agreed to provide the Company with storage space and personnel support at the Azzur Suite. The total estimated project cost during the Term for access to, and use of, the cleanroom and storage space, and the personnel support and other services, is $4.8 million.

The Company may terminate the SOW on four months’ prior written notice at any time during the Term. In addition, either party may terminate the Master Services Agreement (including the SOW) due to a breach by the other party and failure to cure. If the Azzur Suite is not ready for use by the Company as of May 1, 2019, the Company may (i) elect to terminate the SOW, (ii) wait for the Azzur Suite to become available, without incurring any costs (other than a deposit) relating to the Azzur Suite until it becomes available, or (iii) accept an alternate cleanroom space from Azzur on different terms.

In the ordinary course of business, the Company may be subject to legal proceedings, claims and litigation as the Company operates in an industry susceptible to patent legal claims. The Company accounts for estimated losses with respect to legal proceedings and claims when such losses are probable and estimable. Legal costs associated with these matters are expensed when incurred. The Company is not currently a party to any material legal proceedings.

Employee Benefits

12 Months Ended

Dec. 31, 2018

Compensation And Retirement Disclosure [Abstract]

Employee Benefits

(19)	Employee Benefits

The Company has a defined contribution 401(k) plan for eligible employees. Employees are eligible to participate in the plan beginning on their date of hire. Under the terms of the plan, employees may make voluntary contributions as a percentage of compensation. The Company has not made any matching contributions since the adoption of the 401(k) plan.

Related-Party Transactions

12 Months Ended

Dec. 31, 2018

Related-Party Transactions

(20)	Related-Party Transactions

During the year ended December 31, 2018, there were no related-party transactions. During the year ended December 31, 2017, before the Company became a public company, the Company received repayment of the loan to its then-existing chief executive officer of approximately $0.2 million. The loan was repaid in June 2017, including interest which accrued at a rate of 0.6%.

Selected Quarterly Data (unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Data (unaudited)

(21)	Selected Quarterly Data (Unaudited)

The following tables contain quarterly financial information for 2018 and 2017 (in thousands). The Company believes that the following information reflects all normal recurring adjustments necessary for a fair presentation of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.

	2018 Quarter Ended
	March 31			June 30			September 30			December 31
Revenue	$	354		$	254		$	1,801		$	111
Operating expenses		11,990			15,606			13,335		$	12,819
Loss from operations		(11,636	)		(15,352	)		(11,534	)	$	(12,708	)
Net loss		(11,165	)		(14,591	)		(10,748	)	$	(11,931	)
Net loss per share attributable to common stockholders - basic and diluted	$	(0.55	)	$	(0.59	)	$	(0.43	)	$	(0.47	)

	2017 Quarter Ended
	March 31			June 30			September 30			December 31
Revenue	$	111		$	2,111		$	111		$	111
Operating expenses		7,485			11,568			12,186			12,029
Loss from operations		(7,374	)		(9,457	)		(12,075	)		(11,918	)
Net loss		(7,368	)		(9,388	)		(11,924	)		(11,697	)
Net loss per share attributable to common stockholders - basic and diluted	$	—		$	(4.70	)	$	(1.66	)	$	(0.74	)
Net loss per unit attributable to common unit holders - basic and diluted	$	(4.49	)	$	—		$	—		$	—

Subsequent Events

12 Months Ended

Dec. 31, 2018

Subsequent Events [Abstract]

Subsequent Events

(22)	Subsequent Events

On February 28, 2019, the Joint Research Committee related to the Company’s collaboration with AbbVie concluded that the remaining milestone of $2.5 million under the Second Amendment in the Company’s collaboration with AbbVie was achieved. Related revenue will be recognized as the research and development services are provided using an input method, according to the full time equivalents incurred over the third phase of the research plan.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (“U.S.”) (“U.S. GAAP” or “GAAP”).

Principles of Consolidation

Use of Estimates

Cash Equivalents

Concentration of Credit Risk

Restricted Cash

	December 31,		December 31,
	2018		2017
Cash and cash equivalents	$	11,252	$	58,440
Restricted cash included in other long-term assets		1,097		1,097
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$	12,349	$	59,537

Fair Value

•

Level 1 – Utilize observable inputs such as quoted prices in active markets for identical assets or liabilities;

•

Level 2 – Utilize data points that are either directly or indirectly observable, such as quoted prices, interest rates and yield curves;

•

Level 3 – Utilize unobservable data points in which there is little or no market data, which require the Company to develop its own assumptions for the asset or liability.

Available-for-Sale Securities

Property and Equipment

Depreciation begins at the time the asset is placed in service. Depreciation is provided over the following estimated useful lives:

Asset classification		Useful life
Computer and office equipment		3 years
Furniture and fixtures		5 years
Laboratory equipment		5 years
Leasehold improvements		Lesser of useful life or remaining lease term

Impairment of Long-Lived Assets

Impairment of Long‑Lived Assets

Rent Expense

Research and Development Costs

Revenue recognition

The Company generates revenue through a collaboration and license arrangement with a strategic partner for the development and commercialization of product candidates.

Licenses of Intellectual Property

Research and Development Services

Customer Options

Milestone Payments

Royalties

Contract Costs

Equity-Based Compensation

Equity‑Based Compensation

Income Taxes

Net Loss Per Share

Segment Information

Recently Adopted Accounting Pronouncements

Revenue Recognition

Income Taxes

Stock Compensation

Recently Issued Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash

	December 31,		December 31,
	2018		2017
Cash and cash equivalents	$	11,252	$	58,440
Restricted cash included in other long-term assets		1,097		1,097
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$	12,349	$	59,537

Schedule of Useful Life of Property and Equipment

Depreciation begins at the time the asset is placed in service. Depreciation is provided over the following estimated useful lives:

Asset classification		Useful life
Computer and office equipment		3 years
Furniture and fixtures		5 years
Laboratory equipment		5 years
Leasehold improvements		Lesser of useful life or remaining lease term

Merger with Mirna Therapeutics (Tables)

12 Months Ended

Dec. 31, 2018

Business Combinations [Abstract]

Net Assets Acquired in the Merger

The following table shows the net assets acquired in the Merger (in thousands):

	August 28, 2017
Cash and cash equivalents	$	14,882
Marketable securities		27,600
Interest receivable		126
Prepaid assets		112
Unrealized loss on marketable securities		5
Accounts payable and accrued expenses		(105	)
Total net assets acquired		42,620
Less: Transaction costs		(2,187	)
Total net assets acquired less transaction costs	$	40,433

Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Schedule of Company's Classified Assets Measured at Fair Value on Recurring Basis

the Company has classified assets measured at fair value on a recurring basis as follows (in thousands):

	Fair Value Measurements at Reporting Date Using
	December 31,		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
Description	2018		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	265	$	265	$	—	$	—
Corporate debt securities (included in short-term investments)		107,505		—		107,505		—
U.S. government agency securities and treasuries (included in short-term investments)		3,972		1,987		1,985		—
Total	$	111,742	$	2,252	$	109,490	$	—

	Fair Value Measurements at Reporting Date Using
	December 31,		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
Description	2017		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	21,301	$	21,301	$	—	$	—
Corporate debt securities (included in cash and cash equivalents)		11,405		—		11,405		—
Corporate debt securities (included in short-term investments)		28,585		—		28,585		—
Total	$	61,291	$	21,301	$	39,990	$	—

Available-for-Sale Investments (Tables)

12 Months Ended

Dec. 31, 2018

Investments Debt And Equity Securities [Abstract]

Summary of Available-for-Sale Securities Held

The following tables summarize the available-for-sale securities held at December 31, 2018 and 2017 (in thousands):

December 31, 2018	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair Value
Corporate debt securities	$	107,571	$	4	$	(70	)	$	107,505
U.S. government agency securities	$	3,971	$	1	$	—		$	3,972
Total	$	111,542	$	5	$	(70	)	$	111,477

December 31, 2017	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair Value
Corporate debt securities	$	28,593	$	1	$	(9	)	$	28,585
Total	$	28,593	$	1	$	(9	)	$	28,585

Prepaid Expenses and Other Current Assets (Tables)

12 Months Ended

Dec. 31, 2018

Text Block [Abstract]

Schedule of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consists of the following (in thousands):

	December 31,		December 31,
	2018		2017
Prepaid insurance	$	502	$	437
Prepaid research and development		122		508
Other prepaid		597		321
Other current assets		388		298
	$	1,609	$	1,564

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net consists of the following (in thousands):

	December 31,			December 31,
	2018			2017
Laboratory equipment	$	7,111		$	2,999
Computer and office equipment		781			354
Furniture and fixtures		413			220
Leasehold improvements		9,484			2,308
Construction in progress		39			7,017
		17,828			12,898
Less accumulated depreciation		(2,987	)		(3,115	)
	$	14,841		$	9,783

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2018

Payables And Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses consists of the following (in thousands):

	December 31,		December 31,
	2018		2017
Payroll related	$	2,906	$	1,721
Professional fees		306		805
Research and development		1,585		2,027
Other		237		270
	$	5,034	$	4,823

Equity-based Compensation and Equity Incentive Plans (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Restricted Common Stock Activity

The following table shows restricted common stock activity:

	Restricted stock awards
				Weighted average grant date
	Number of			fair value
	shares			(per share)
Unvested at December 31, 2016		—		$	—
Awards exchanged upon 2017 Reorganization		1,059,912			13.53
Granted		2,884			19.01
Vested		(671,204	)		13.54
Forfeited		(16,113	)		13.53
Unvested at December 31, 2017		375,479		$	13.55
Granted		—			—
Vested		(186,332	)		13.54
Forfeited		(70,468	)		13.61
Unvested at December 31, 2018		118,679		$	13.54

Schedule of Equity-based Compensation Expenses

	Years ended December 31,
	2018		2017
Research and development	$	1,333	$	1,410
General and administrative		2,984		1,242
	$	4,317	$	2,652

Schedule of Equity-based Compensation Expenses by Award Type

The following table summarizes equity‑based compensation expense by type of award for the years ended December 31, 2018 and 2017 (in thousands):

	Years ended December 31,
	2018		2017
Stock options	$	3,361	$	1,956
Restricted stock awards		956		508
Incentive units		—		132
Restricted common units		—		56
	$	4,317	$	2,652

Restricted Common Units

Schedule of Restricted Common Unit Activity

The following table shows the restricted common unit activity for the year ended December 31, 2017, prior to the 2017 Reorganization, as adjusted for the Merger:

	Restricted common units
				Grant date
	Number of			fair value
	units			(per unit)
Unvested at December 31, 2016		219,087		$	1.48
Granted		—			—
Vested		(37,770	)		1.48
Forfeited		—			—
Exchanged as part of 2017 Reorganization		(181,317	)		1.48
Unvested at December 31, 2017		—		$	—

2015 and 2017 Plan

Schedule of Weighted Average Assumption Used Black-Scholes Option-pricing Model for Stock Options Issued to Employees and Nonemployees

	Year ended December 31,
Employees:	2018			2017
Expected term	6.2 years			6.2 years
Weighted-average, risk-free interest rate		2.8	%		2.1	%
Expected volatility		70.8	%		70.4	%
Dividend yield		—			—

Schedule of Stock Option Activity

The following table summarizes stock option activity, as adjusted for the Exchange Ratio under the 2015 and 2017 Plans.

	Stock options outstanding
						Weighted
						average
				Weighted		remaining		Aggregate
				average		contractual		intrinsic
	Number of			exercise		term		value^(a)
	options			price		(in years)		(in thousands)
Outstanding at December 31, 2017		1,267,221		$	13.62		9.6	$	—
Granted		919,496			9.71
Exercised		(19,830	)		12.28
Forfeited		(394,136	)		12.09
Expired		(32,867	)		13.53
Outstanding at December 31, 2018		1,739,884			11.92		9.0	$	—

Vested or expected to vest at December 31, 2018		1,739,884			11.92		9.0	$	—
Exercisable at December 31, 2018		489,236			13.20		8.4	$	—

^(a)	The aggregate intrinsic value is calculated as the difference between the exercise price of the options and the fair market value of the underlying common stock for the options that were in the money at December 31, 2018 . No options were in the money at December 31, 2018 .

Plan 2015

Schedule of Stock Option Activity

The following table represents a summary of incentive unit activity, as adjusted for the Merger, under the 2015 LLC Plan:

	Incentive units
				Weighted-		Weighted-		Weighted-
				average		average		average
	Number of			strike		threshold		grant date
	units			price		price		fair value
Non-vested units at December 31, 2016		971,906		$	5.22	$	5.93	$	1.01
Granted		—			—		—		—
Vested		(73,719	)		4.01		5.53		0.87
Forfeited		(260,145	)		4.19		5.57		1.05
Non-vested units cancelled upon 2017 Reorganization		(638,042	)		5.78		6.15		1.05
Non-vested units at December 31, 2017		—		$	—	$	—	$	—

Vested or expected to vest at December 31, 2017		—		$	—	$	—	$	—

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Schedule of Computation of Basic and Diluted Net Loss per Share

The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders (in thousands, except for share and per share amounts):

	2018			2017
Numerator:
Net loss attributable to common stockholders	$	(48,435	)	$	(40,377	)
Denominator:
Weighted-average common shares outstanding - basic and diluted		23,882,685			6,724,641
Net loss per share attributable to common stockholders - basic and diluted	$	(2.03	)	$	(6.00	)

Schedule of Potentially Common Shares Excluded from Calculation of Net Loss Per share

	As of December 31,
	2018		2017
Unvested restricted common stock awards		118,679		375,479
Outstanding options to purchase common stock		1,739,884		1,267,221

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Provision for Income Tax

The provision for income taxes consist of the following (in thousands):

	December 31,
	2018		2017
Current Tax Expense:
Federal	$	—	$	—
State		33		—
	$	33	$	—

Schedule of Temporary Differences Between Basis of Deferred Tax Assets and Liabilities

	December 31,
	2018			2017
Deferred tax assets:
Net operating loss carryforwards	$	33,275		$	21,248
Tax credit carryforwards		4,365			3,038
Accrued expenses		103			32
Property and equipment		—			390
Deferred rent		2,209			53
Equity compensation		784			503
Amortizable intangibles		1,339			1,492
Other		78			—
Gross deferred tax assets		42,153			26,756
Deferred tax liability:
Other		—			(241	)
Property and equipment		(1,863	)		—
Valuation allowance		(40,290	)		(26,515	)
Net deferred tax assets	$	—		$	—

Reconciliation of Statutory Federal Income Tax Rate to Company's Effective Income Tax Rate

A reconciliation of the statutory federal income tax rate to the Company’s effective income tax rate is as follows (dollars in thousands):

	Years ended December 31,
	2018			2017
	Tax Rate			Tax Rate
U.S. federal statutory rate		21	%		34	%
State income taxes, net of federal benefit		6	%		5	%
Other permanent differences		(1	)%		(1	)%
Tax credits		3	%		5	%
Other items		(1	)%		—
Change in rate due to Tax Reform		—			(27	)%
Mirna acquisition		—			17	%
Net change in valuation allowance		(28	)%		(33	)%
Effective income tax rate		—			—

Schedule of Valuation Allowance

A roll-forward of the valuation allowance for the years ended December 31, 2018 and 2017 is as follows (in thousands):

	Years ended December 31,
	2018			2017
Balance at beginning of year	$	(26,515	)	$	(13,060	)
Increase in valuation allowance		(13,775	)		(13,455	)
Balance at end of year	$	(40,290	)	$	(26,515	)

Leases (Tables)

12 Months Ended

Dec. 31, 2018

Leases [Abstract]

Schedule of Future Minimum Lease Payments under Non-cancelable Operating and Capital Leases

Future minimum lease payments under the Company’s non-cancelable operating and capital leases as of December 31, 2018, are as follows (in thousands):

	Operating		Capital
	leases		leases
Fiscal year:
2019	$	3,175	$	287
2020		3,270		214
2021		3,369		2
2022		3,470		—
2023		3,574		—
Thereafter		18,067		—
Total future minimum lease payments	$	34,925	$	503
Less amounts representing interest				27
Capital lease obligations at December 31, 2018				476
Less current portion of capital lease obligations				266
Capital lease obligations, net of current portion			$	210

Selected Quarterly Data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Data (unaudited)

	2018 Quarter Ended
	March 31			June 30			September 30			December 31
Revenue	$	354		$	254		$	1,801		$	111
Operating expenses		11,990			15,606			13,335		$	12,819
Loss from operations		(11,636	)		(15,352	)		(11,534	)	$	(12,708	)
Net loss		(11,165	)		(14,591	)		(10,748	)	$	(11,931	)
Net loss per share attributable to common stockholders - basic and diluted	$	(0.55	)	$	(0.59	)	$	(0.43	)	$	(0.47	)

	2017 Quarter Ended
	March 31			June 30			September 30			December 31
Revenue	$	111		$	2,111		$	111		$	111
Operating expenses		7,485			11,568			12,186			12,029
Loss from operations		(7,374	)		(9,457	)		(12,075	)		(11,918	)
Net loss		(7,368	)		(9,388	)		(11,924	)		(11,697	)
Net loss per share attributable to common stockholders - basic and diluted	$	—		$	(4.70	)	$	(1.66	)	$	(0.74	)
Net loss per unit attributable to common unit holders - basic and diluted	$	(4.49	)	$	—		$	—		$	—

Nature of Business - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Apr. 30, 2018	Jan. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Nature Of Business [Line Items]
Cash, cash equivalents, and marketable securities			$ 122,700
Restricted cash			1,097	$ 1,097
Accumulated deficit			(119,765)	$ (71,654)
Proceeds from sale of common stock, net of issuance costs	$ 28,900	$ 53,800	$ 82,666
Common Stock \| Underwritten Public Offering, Including Underwriters’ Allotment
Nature Of Business [Line Items]
Proceeds from sale of common stock, net of issuance costs		$ 53,800
Common Stock \| Direct Offering
Nature Of Business [Line Items]
Proceeds from sale of common stock, net of issuance costs	$ 28,900

Summary of Significant Accounting Policies - Additional Information (Detail)	1 Months Ended	3 Months Ended								12 Months Ended
	Jul. 31, 2015	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)	Aug. 28, 2017
Summary Of Significant Accounting Policies [Line Items]
Revenue, practical expedient, incremental cost of obtaining contract [true/false]										true
Revenue, practical expedient, remaining performance obligation, description										The Company recognizes as an asset the incremental costs of obtaining a contract with a customer if the costs are expected to be recovered. As a practical expedient, the Company recognizes the incremental costs of obtaining a contract as an expense when incurred if the amortization period of the asset that we otherwise would have recognized is one year or less. To date, the Company has not incurred any incremental costs of obtaining a contract with a customer
Cash equivalents maturity										three months or less
Cash equivalents		$ 300,000				$ 32,700,000				$ 300,000	$ 32,700,000
Cash held in a separate restricted bank account		1,097,000				1,097,000				1,097,000	1,097,000
Transfers of assets and liabilities between Level 1, Level 2, or Level 3										0	0
Impairment charge										$ 0
Lease expiration date	2021-04									2017-07
Incentive unit liquidation value		0								$ 0
Description of tax benefit likely to be realized upon settlement										greater than 50%
Exchange ratio of common stock													0.5532
Number of operating segment \| Segment										1
Revenue		111,000	$ 1,801,000	$ 254,000	$ 354,000	111,000	$ 111,000	$ 2,111,000	$ 111,000	$ 2,520,000	2,444,000
ASC 606
Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect decrease to opening accumulated deficit												$ 300,000
Deferred revenue		300,000								300,000
Revenue		2,500,000
ASC 606 \| Impact as Result of Adoption of ASC 606
Summary Of Significant Accounting Policies [Line Items]
Deferred revenue		(300,000)								(300,000)
ASC 606 \| Balances without Adoption of ASC 606
Summary Of Significant Accounting Policies [Line Items]
Deferred revenue		700,000								700,000
Revenue		2,400,000
Bank Account
Summary Of Significant Accounting Policies [Line Items]
Cash held in a separate restricted bank account		50,000				$ 50,000				50,000	$ 50,000
Letter of Credit
Summary Of Significant Accounting Policies [Line Items]
Cash		$ 1,000,000								$ 1,000,000

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 11,252	$ 58,440
Restricted cash included in other long-term assets	1,097	1,097
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$ 12,349	$ 59,537	$ 14,636

Summary of Significant Accounting Policies - Schedule of Useful Life of Property and Equipment (Detail)	12 Months Ended
	Dec. 31, 2018
Computer and Office Equipment
Property Plant And Equipment [Line Items]
Useful life	3 years
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Useful life	5 years
Laboratory Equipment
Property Plant And Equipment [Line Items]
Useful life	5 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Useful life	Lesser of useful life or remaining lease term

Merger with Mirna Therapeutics - Additional Information (Detail) $ in Millions	Aug. 28, 2017 USD ($) Director shares	Dec. 31, 2018 shares	Dec. 31, 2017 shares
Business Acquisition [Line Items]
Merger completion date	Aug. 28, 2017
Number of members on Board of Directors \| Director	7
Exchange ratio of common stock	0.5532
Common stock outstanding \| shares		25,401,479	16,272,617
Synlogic
Business Acquisition [Line Items]
Number of members on Board of Directors \| Director	5
Mirna therapeutics
Business Acquisition [Line Items]
Common stock outstanding \| shares	20,900,000
Market capitalization \| $	$ 35.0
Fair value, net asset (Liability) \| $	$ 42.6
Mirna therapeutics \| Common Stock \| Synlogic
Business Acquisition [Line Items]
Share ownership following merger, percent	83.00%

Merger with Mirna Therapeutics - Net Assets Acquired in the Merger (Detail) - Mirna therapeutics $ in Thousands	Aug. 28, 2017 USD ($)
Business Acquisition [Line Items]
Cash and cash equivalents	$ 14,882
Marketable securities	27,600
Interest receivable	126
Prepaid assets	112
Unrealized loss on marketable securities	5
Accounts payable and accrued expenses	(105)
Total net assets acquired	42,620
Less: Transaction costs	(2,187)
Total net assets acquired less transaction costs	$ 40,433

Fair Value of Financial Instruments - Schedule of Company's Classified Assets Measured at Fair Value on Recurring Basis (Detail) - Recurring - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Asset Measured On Recurring Basis [Line Items]
Total assets	$ 111,742	$ 61,291
U.S. Government Agency Securities and Treasuries
Fair Value Asset Measured On Recurring Basis [Line Items]
Short-term investments	3,972
Corporate debt securities
Fair Value Asset Measured On Recurring Basis [Line Items]
Cash and Cash Equivalents		11,405
Short-term investments	107,505	28,585
Money market funds
Fair Value Asset Measured On Recurring Basis [Line Items]
Cash and Cash Equivalents	265	21,301
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value Asset Measured On Recurring Basis [Line Items]
Total assets	2,252	21,301
Quoted Prices in Active Markets for Identical Assets (Level 1) \| U.S. Government Agency Securities and Treasuries
Fair Value Asset Measured On Recurring Basis [Line Items]
Short-term investments	1,987
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Money market funds
Fair Value Asset Measured On Recurring Basis [Line Items]
Cash and Cash Equivalents	265	21,301
Significant Other Observable Inputs (Level 2)
Fair Value Asset Measured On Recurring Basis [Line Items]
Total assets	109,490	39,990
Significant Other Observable Inputs (Level 2) \| U.S. Government Agency Securities and Treasuries
Fair Value Asset Measured On Recurring Basis [Line Items]
Short-term investments	1,985
Significant Other Observable Inputs (Level 2) \| Corporate debt securities
Fair Value Asset Measured On Recurring Basis [Line Items]
Cash and Cash Equivalents		11,405
Short-term investments	107,505	28,585
Significant Unobservable Inputs (Level 3)
Fair Value Asset Measured On Recurring Basis [Line Items]
Total assets	0	0
Significant Unobservable Inputs (Level 3) \| U.S. Government Agency Securities and Treasuries
Fair Value Asset Measured On Recurring Basis [Line Items]
Short-term investments	0
Significant Unobservable Inputs (Level 3) \| Corporate debt securities
Fair Value Asset Measured On Recurring Basis [Line Items]
Cash and Cash Equivalents		0
Short-term investments	0	0
Significant Unobservable Inputs (Level 3) \| Money market funds
Fair Value Asset Measured On Recurring Basis [Line Items]
Cash and Cash Equivalents	$ 0	$ 0

Available-for-Sale Investments - Summary of Available-for-Sale Securities Held (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 111,542	$ 28,593
Gross unrealized gains	5	1
Gross unrealized losses	(70)	(9)
Fair Value	111,477	28,585
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	107,571	28,593
Gross unrealized gains	4	1
Gross unrealized losses	(70)	(9)
Fair Value	107,505	$ 28,585
U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	3,971
Gross unrealized gains	1
Fair Value	$ 3,972

Available-for-Sale Investments - Additional Information (Detail) $ in Millions	Dec. 31, 2018 USD ($) Investment Security	Dec. 31, 2017 USD ($)
Investments Debt And Equity Securities [Abstract]
Number of investments in unrealized loss position	37
Number of investments in unrealized loss position, more than twelve months	0
Aggregate fair value of securities in unrealized loss position \| $	$ 96.5	$ 19.3
Number of securities with other than temporary impairment \| Security	0

Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid insurance	$ 502	$ 437
Prepaid research and development	122	508
Other prepaid	597	321
Other current assets	388	298
Total	$ 1,609	$ 1,564

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 17,828	$ 12,898
Less accumulated depreciation	(2,987)	(3,115)
Property and equipment, net	14,841	9,783
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	7,111	2,999
Computer and Office Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	781	354
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	413	220
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	9,484	2,308
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 39	$ 7,017

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Leasehold Improvements
Property Plant And Equipment [Line Items]
Lessor-paid tenant improvements	$ 6.6	$ 1.3
Construction in Progress
Property Plant And Equipment [Line Items]
Lessor-paid tenant improvements		$ 5.0
Laboratory Equipment
Property Plant And Equipment [Line Items]
Minimum percentage of fair value on present value of expected payments	90.00%	90.00%
Assets under capital lease	$ 1.3	$ 1.4
Accumulated depreciation of capital lease	$ 0.9	$ 0.2

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Payroll related	$ 2,906	$ 1,721
Professional fees	306	805
Research and development	1,585	2,027
Other	237	270
Total accrued expenses	$ 5,034	$ 4,823

Common Stock - Additional Information (Detail)	1 Months Ended		12 Months Ended	20 Months Ended	133 Months Ended
Common Stock - Additional Information (Detail)	Apr. 30, 2018 USD ($) $ / shares shares	Jan. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) Vote shares	Dec. 31, 2018 shares	Dec. 31, 2018 USD ($) shares
Class Of Stock [Line Items]
Number of votes entitled to each share of common stock \| Vote			1
Cash dividends \| $					$ 0
Proceeds from sale of common stock, net of issuance costs \| $	$ 28,900,000	$ 53,800,000	$ 82,666,000
Common Stock
Class Of Stock [Line Items]
Unvested shares of common stock			118,679	118,679	118,679
Number of shares forfeited				86,581
Repurchase option exercisable period			4 years
Repurchase options share exercised				41,819
Stock sold and issued to investors	3,280,000	5,899,500
Stock sold and issued to investors, per share \| $ / shares	$ 9.15	$ 9.75

Preferred Stock - Additional Information (Detail)	1 Months Ended				12 Months Ended
Preferred Stock - Additional Information (Detail)	May 31, 2017 USD ($) $ / shares shares	Feb. 29, 2016 shares	May 31, 2015 shares	Sep. 30, 2014 shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Aug. 28, 2017
Class Of Stock [Line Items]
Exchange ratio of common stock							0.5532
Preferred stock, conversion basis					one-for-one
Preferred stock, conversion percentage					100.00%
Total consideration from stock sold and issued to investors \| $						$ 40,433,000
Preferred stock dividends declared					$ 0	$ 0
Preferred stock dividends paid					0	0
Preferred stock, par value					0.001	$ 0.001
Public offering price per share threshold for conversion of preferred stock					$ 12.09
Aggregate proceeds threshold for conversion of preferred stock \| $					$ 50,000,000
Preferred stock, shares issued \| shares					0	0
Letter Agreement \| The Gates Foundation
Class Of Stock [Line Items]
Minimum percentage of initial public offering over price paid upon redemption						200.00%
Private Synlogic
Class Of Stock [Line Items]
Preferred stock, par value					$ 0.0001
Series C Preferred Stock
Class Of Stock [Line Items]
Stock sold and issued to investors \| shares	2,882,679
Stock sold and issued to investors, per share	$ 8.06
Total consideration from stock sold and issued to investors \| $	$ 40,400,000
Offering costs \| $	$ 1,600,000
Series A Preferred Stock
Class Of Stock [Line Items]
Vote of preferred stock holders required for approval					70.00%
Series A Preferred Stock \| Letter Agreement \| The Gates Foundation
Class Of Stock [Line Items]
Stock sold and issued to investors \| shares		361,884	218,646	201,163
Preferred stock, shares issued \| shares				781,693
Proceeds from issuance of stock to be spend on research \| $						$ 5,000,000

Preferred Units - Additional Information (Detail) - USD ($)	1 Months Ended				12 Months Ended
	Feb. 29, 2016	Jul. 31, 2015	May 31, 2015	Sep. 30, 2014	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class Of Stock [Line Items]
Net proceeds from issue and sale of preferred units						$ 26,648,000
Distribution paid					$ 0	$ 0
Preferred stock, shares issued					0	0
Minimum
Class Of Stock [Line Items]
Percentage of preferred unit holders					70.00%
Letter Agreement \| The Gates Foundation \| Series A Preferred Stock
Class Of Stock [Line Items]
Preferred stock, shares issued				781,693
Preferred stock, shares issued	361,884		218,646	201,163
Shares exchanged for preferred units		419,809
Letter Agreement \| The Gates Foundation \| Contingently Redeemable Class A Preferred Units
Class Of Stock [Line Items]
Preferred units classified as temporary equity							781,693
Preferred units classified as temporary equity, cost							$ 5,000,000
Class A-3 Preferred Units
Class Of Stock [Line Items]
Units issued and sold during the period	2,005,348
Units sold and issued to investors, per share	$ 7.23
Contingently Redeemable Class A-3 Preferred Stock
Class Of Stock [Line Items]
Units issued and sold during the period	361,884
Units sold and issued to investors, per share	$ 7.23
Class A-3 Preferred Units and Contingently Redeemable Class A-3 Preferred Stock
Class Of Stock [Line Items]
Net proceeds from issue and sale of preferred units	$ 17,100,000
Issuance costs	$ 0
Class B Preferred Units
Class Of Stock [Line Items]
Units issued and sold during the period	1,029,850
Units sold and issued to investors, per share	$ 13.53
Net proceeds from issue and sale of preferred units	$ 13,600,000
Issuance costs	$ 300,000					$ 18,000

Equity-based Compensation and Equity Incentive Plans - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Aug. 28, 2017 shares	Aug. 31, 2015 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exchange ratio of common stock			0.5532
Equity-based compensation expense	$ 4,317,000	$ 2,652,000
Weighted average grant date fair value per share of options granted to employees \| $ / shares	$ 6.35
Total fair value of awards vested	$ 3,300,000
Employee unrecognized compensation expense	$ 8,400,000
Employee unrecognized compensation cost, period of recognition	2 years 7 months 6 days
Non-employee unrecognized compensation expense	$ 53,000,000
Non-employee unrecognized compensation cost, period of recognition	2 years 4 months 24 days
Restricted Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense	$ 956,000	$ 508,000
Employee unrecognized compensation expense	$ 200,000
Employee unrecognized compensation cost, period of recognition	1 year 7 months 6 days
Non-employee unrecognized compensation expense	$ 0
Granted \| shares	0	1,062,794
Vested \| shares	186,332	671,204
Restricted Common Stock \| 2017 Reorganization
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted \| shares		1,059,912
Incentive Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense		$ 132,000
Employee unrecognized compensation expense	$ 0
Units issued \| shares	0	0
Restricted Common Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense		$ 56,000
Vested \| shares		37,770
Unrecognized compensation expense	$ 0
Private Synlogic \| Restricted Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense	$ 26,000
Employees and Nonemployees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted \| shares	919,496	1,281,647
Equity-based compensation expense	$ 3,200,000	$ 2,000,000
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of discount for employees under ESPP	15.00%
Purchase price as a percentage of fair value under ESPP	85.00%
Plan 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock option and stock purchase rights available for grant (in shares) \| shares				647,893
Percentage of shares outstanding	5.00%
Award vesting description	Incentive units issued by Synlogic, LLC under the 2015 LLC Plan generally vested 25% after one year and ratably monthly thereafter over the next 36 months.
Options vested period	36 months
Plan 2015 \| After One Year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options awards, vesting percentage	25.00%
2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock option and stock purchase rights available for grant (in shares) \| shares			1,753,061
2017 Plan \| Restricted Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted \| shares		2,884
2015 and 2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grants \| shares	477,414
Granted \| shares	919,496
Modifications in Equity Awards \| Separation of Former Chief Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation	$ 700,000

Equity-based Compensation and Equity Incentive Plans - Schedule of Weighted Average Assumption Used Black-Scholes Option-pricing Model for Stock Options Issued to Employees and Nonemployees (Detail) - 2015 and 2017 Plan	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	6 years 2 months 12 days	6 years 2 months 12 days
Weighted-average, risk-free interest rate	2.80%	2.10%
Expected volatility	70.80%	70.40%
Dividend yield	0.00%	0.00%
Nonemployee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term		6 years 2 months 12 days
Weighted-average, risk-free interest rate		2.10%
Expected volatility		70.40%
Dividend yield		0.00%

Equity-based Compensation and Equity Incentive Plans - Schedule of Stock Option Activity Under 2015 and 2017 Plan (Detail) - 2015 and 2017 Plan - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding Beginning balance, Number of options	1,267,221
Granted, Number of options	919,496
Exercised, Number of options	(19,830)
Forfeited, Number of options	(394,136)
Expired, Number of options	(32,867)
Outstanding Ending balance, Number of options	1,739,884	1,267,221
Number of options, Vested or expected to vest	1,739,884
Number of options, Exercisable	489,236
Beginning balance, Weighted-average price	$ 13.62
Granted, Weighted-average price	9.71
Exercised, Weighted-average price	12.28
Forfeited, Weighted-average price	12.09
Expired, Weighted-average price	13.53
Ending balance, Weighted-average price	11.92	$ 13.62
Weighted-average price Vested or expected to vest	11.92
Weighted-average price, Exercisable	$ 13.20
Outstanding, weighted average remaining contractual term (Year)	9 years	9 years 7 months 6 days
Weighted average remaining contractual term, Vested or expected to vest	9 years
Weighted average remaining contractual term, Exercisable	8 years 4 months 24 days

Equity-based Compensation and Equity Incentive Plans - Schedule of Restricted Common Stock Activity (Detail) - Restricted Common Stock - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Beginning balance, Number of unvested shares/units	375,479
Granted, Number of shares/units	0	1,062,794
Vested, Number of shares/units	(186,332)	(671,204)
Forfeited, Number of shares/units	(70,468)	(16,113)
Ending balance, Number of unvested shares/units	118,679	375,479
Beginning balance, Unvested Grant date fair value	$ 13.55
Vested, Grant date fair value	13.54	$ 13.54
Forfeited, Grant date fair value	13.61	13.53
Ending balance, Unvested Grant date fair value	$ 13.54	$ 13.55
2017 Reorganization
Granted, Number of shares/units		1,059,912
Granted, Grant date fair value		$ 13.53
2017 Plan
Granted, Number of shares/units		2,884
Granted, Grant date fair value		$ 19.01

Equity-based Compensation and Equity Incentive Plans - Schedule of Stock Option Activity Under 2015 LLC Plan (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Weighted-average grant date fair value	$ 6.35
Plan 2015 \| Incentive Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, Number of Non-vested Units	0	971,906
Vested, Number of Units		(73,719)
Forfeited, Number of Units		(260,145)
Ending balance, Number of Non-vested Units		0
Vested or expected to vest, Number of Units		0
Beginning balance, Weighted-average price	$ 0	$ 5.22
Vested, Weighted-average price		4.01
Forfeited, Weighted-average price		4.19
Ending balance, Weighted-average price		0
Weighted-average price, Vested or expected to vest at December 31, 2017		0
Beginning Balance, Weighted-average grant date fair value	0	1.01
Vested, Weighted-average grant date fair value		0.87
Forfeited, Weighted-average grant date fair value		1.05
Ending balance, Weighted-average grant date fair value		0
Weighted-average grant date fair value, Vested or expected to vest at December 31, 2017		0
Plan 2015 \| Incentive Units \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, Weighted-average price	$ 0	5.93
Granted, Weighted-average price		0
Vested, Weighted-average price		5.53
Forfeited, Weighted-average price		5.57
Ending balance, Weighted-average price		0
Weighted-average price, Vested or expected to vest at December 31, 2017		$ 0
Plan 2015 \| Incentive Units \| 2017 Reorganization
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested units cancelled upon 2017 Reorganization, Number of Units		(638,042)
Non-vested units cancelled upon 2017 Reorganization, Weighted-average price		$ 5.78
Non-vested units cancelled upon 2017 Reorganization, Weighted-average grant date fair value		1.05
Plan 2015 \| Incentive Units \| 2017 Reorganization \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested units cancelled upon 2017 Reorganization, Weighted-average price		$ 6.15

Equity-based Compensation and Equity Incentive Plans - Schedule of Restricted Common Unit Activity (Detail) - Restricted Common Units	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Beginning balance, Number of unvested shares/units \| shares	219,087
Vested, Number of shares/units \| shares	(37,770)
Beginning balance, Unvested Grant date fair value \| $ / shares	$ 1.48
Vested, Grant date fair value \| $ / shares	$ 1.48
2017 Reorganization
Exchanged as part of 2017 Reorganization, Number of shares/units \| shares	(181,317)
Exchanged as part of 2017 Reorganization, Grant date fair value \| $ / shares	$ 1.48

Equity-based Compensation and Equity Incentive Plans - Schedule of Equity-based Compensation Expenses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense	$ 4,317	$ 2,652
Research and Development Expense
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense	1,333	1,410
General and Administrative Expense
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense	$ 2,984	$ 1,242

Equity-based Compensation and Equity Incentive Plans - Schedule of Equity-based Compensation Expenses by Award Type (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense	$ 4,317	$ 2,652
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense	3,361	1,956
Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense	$ 956	508
Incentive Units
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense		132
Restricted Common Units
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense		$ 56

Distributions - Additional Information (Detail)	12 Months Ended
Distributions - Additional Information (Detail)	Dec. 31, 2018 USD ($)
Distributions [Abstract]
Earnings distributed	$ 0

Significant Agreements - Additional Information (Detail)			1 Months Ended				3 Months Ended								12 Months Ended
	Dec. 18, 2018	Sep. 27, 2018 USD ($) Milestone	Sep. 30, 2018 USD ($)	May 31, 2017 USD ($)	Apr. 30, 2017 USD ($) shares	Dec. 31, 2015 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Increase (decrease) in deferred revenue															$ (520,000)	$ (444,000)
Recognition of revenue							$ 111,000	$ 1,801,000	$ 254,000	$ 354,000	$ 111,000	$ 111,000	$ 2,111,000	$ 111,000	2,520,000	2,444,000
Deferred revenue included in current liabilities							268,000				$ 444,000				268,000	444,000
AbbVie Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of mile stone payments under second phase \| Milestone		2
Initial milestone payment upon execution of amendment		$ 2,000,000
Increase in transaction price of performance obligation of second phase		$ 2,000,000
Cumulative catch-up adjustment to revenue, contract modification			$ 1,800,000
Increase (decrease) in deferred revenue			$ 300,000
Recognition of revenue															2,500,000	$ 2,400,000
Deferred revenue included in current liabilities							$ 300,000								300,000
AbbVie Agreement \| IBDCo
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront non refundable payment						$ 2,000,000
Potential milestone payment															$ 16,500,000
Collaboration in research and development															54 months
Milestone payment				$ 2,000,000
Massachusetts Institute of Technology and Boston University License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Type of Cost, Good or Service [Extensible List]					us-gaap:LicenseAndServiceMember
Recognition of license fees					$ 1,800,000
Payment for prior patent costs					300,000
Massachusetts Institute of Technology and Boston University License Agreement \| Intellectual Property
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License acquired					$ 50,000
Massachusetts Institute of Technology and Boston University License Agreement \| Common Stock \| 2017 Reorganization
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, converted \| shares					325,377
Massachusetts Institute of Technology and Boston University License Agreement \| Common units
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common units issued for license \| shares					325,377
Massachusetts Institute of Technology and Boston University License Agreement \| Mirna therapeutics \| Common Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, converted \| shares					179,999
Boston University License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement termination effective date	Feb. 16, 2019
Massachusetts Institute of Technology License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement termination effective date	Jun. 19, 2019

Net Loss per Share - Schedule of Computation of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss attributable to common stockholders	$ (11,931)	$ (10,748)	$ (14,591)	$ (11,165)	$ (11,697)	$ (11,924)	$ (9,388)	$ (7,368)	$ (48,435)	$ (40,377)
Denominator:
Weighted-average common shares outstanding - basic and diluted									23,882,685	6,724,641
Net loss per share attributable to common shareholders - basic and diluted	$ (0.47)	$ (0.43)	$ (0.59)	$ (0.55)	$ (0.74)	$ (1.66)	$ (4.70)		$ (2.03)	$ (6.00)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Net Loss per Share - Schedule of Potentially Common Shares Excluded from Calculation of Net Loss Per share (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Unvested Restricted Common Stock Awards
Potentially Common Shares Excluded from Calculation of Net Loss Per share	118,679	375,479
Stock Options
Potentially Common Shares Excluded from Calculation of Net Loss Per share	1,739,884	1,267,221

Income Taxes - Schedule of Provision for Income Tax (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Current Tax Expense:
State	$ 33
Total current tax expense	$ 33

Income Taxes - Schedule of Temporary Differences Between Basis of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 33,275	$ 21,248
Tax credit carryforwards	4,365	3,038
Accrued expenses	103	32
Property and equipment		390
Deferred rent	2,209	53
Equity compensation	784	503
Amortizable intangibles	1,339	1,492
Other	78
Gross deferred tax assets	42,153	26,756
Deferred tax liability:
Other		(241)
Property and equipment	(1,863)
Valuation allowance	$ (40,290)	$ (26,515)	$ (13,060)

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Line Items]
Valuation allowance	$ 40,290,000	$ 26,515,000	$ 13,060,000
Description of tax benefit likely to be realized upon settlement	greater than 50%
Unrecognized tax benefits	$ 0
Income tax examination description	In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. Tax years from 2015 to the present are open to examination under the statute.
Interest or penalties accrued	$ 0	0
Income tax cuts and jobs act provisional amounts measurement period	1 year
Adjustment of deferred tax asset and liabilities due to tax cuts and jobs act	$ 0
Net impact of revaluation of deferred tax assets and liabilities	0
Federal
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 125,500,000	$ 82,000,000
Net operating loss carryforwards expiration year	2034	2034
Federal \| Research and Development.
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 2,800,000
Tax credit carryforwards, expiration year	2034
State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 125,500,000	$ 82,000,000
Net operating loss carryforwards expiration year	2034	2034
State \| Research and Development.
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 1,600,000
Tax credit carryforwards, expiration year	2029

Income Taxes - Reconciliation of Statutory Federal Income Tax Rate to Company's Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Line Items]
U.S. federal statutory rate	21.00%	34.00%
State income taxes, net of federal benefit, Tax Rate	6.00%	5.00%
Other permanent differences, Tax Rate	(1.00%)	(1.00%)
Tax credits, Tax Rate	3.00%	5.00%
Other items, Tax Rate	(1.00%)
Change in rate due to Tax Reform, Tax Rate		(27.00%)
Net change in valuation allowance, Tax Rate	(28.00%)	(33.00%)
Mirna therapeutics
Income Tax Disclosure [Line Items]
Mirna acquisition, Tax Rate		17.00%

Income Taxes - Schedule of Valuation Allowance - (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ (26,515)	$ (13,060)
Increase in valuation allowance	(13,775)	(13,455)
Balance at end of year	$ (40,290)	$ (26,515)

Leases - Additional Information (Detail)	1 Months Ended		12 Months Ended
Leases - Additional Information (Detail)	Jul. 31, 2017 USD ($) ft²	Jul. 31, 2015 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($) Agreement	Oct. 31, 2016 USD ($)
Lessee Lease Description [Line Items]
Operating Lease Annual Rent			$ 2,500,000
Rent credit due to accelerated amortization of deferred rent				$ 200,000
Remaining amortization period of deferred rent and remaining useful life leasehold improvements	63 months
Revised amortization period of deferred rent and revised useful life leasehold improvements	7 months	7 months
Laboratory and office space to be leased \| ft²	41,346
Operating lease annual rent	$ 3,100,000	$ 900,000
Term of lease	10 years	63 months
Tenant improvement investment	$ 1,600,000
Capitalized landlord tenant improvement funded			$ 5,000,000
Letter of credit	1,000,000
Lease commencement date		2016-02
Lease expiration date		2021-04	2017-07
Renewal term of lease		1 year
Agreement termination term		30 days
Penalty on contract termination		$ 0
Yearly increases in base rent		3.00%
Rent abatement term		3 months
Number of non cancellable lease agreements \| Agreement					2
Non cancellable lease agreements description			the present value of the lease exceeds 90% of the fair market value.
Agreement termination			2018-11
Non-cancellable Lease Agreement Three
Lessee Lease Description [Line Items]
Term of non-cancellable lease					36 months
Non-cancellable lease					$ 200,000
Interest rate on outstanding capital lease obligation			1.10%
Non-cancellable Lease Agreement Four
Lessee Lease Description [Line Items]
Term of non-cancellable lease					36 months
Non-cancellable lease					$ 700,000
Interest rate on outstanding capital lease obligation			7.30%
Non Cancellable Lease Agreement Two
Lessee Lease Description [Line Items]
Term of non-cancellable lease						24 months
Non-cancellable lease						$ 400,000
Interest rate on outstanding capital lease obligation				9.60%
Maximum
Lessee Lease Description [Line Items]
Tenant improvement investment	$ 6,600,000

Leases - Schedule of Future Minimum Lease Payments under Non-cancelable Operating and Capital Leases (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Operating leases
2019	$ 3,175
2020	3,270
2021	3,369
2022	3,470
2023	3,574
Thereafter	18,067
Total future minimum lease payments	34,925
Capital leases
2019	287
2020	214
2021	2
Total future minimum lease payments	503
Less amounts representing interest	27
Capital lease obligations at December 31, 2018	476
Less current portion of capital lease obligations	266	$ 425
Capital lease obligations, net of current portion	$ 210	$ 466

Commitments and Contingencies - Additional Information (Detail) - Master Services Agreement - Azzur Group, LLC $ in Millions	Dec. 07, 2018 USD ($) ft²
Commitments And Contingencies [Line Items]
Access and use of space under agreement \| ft²	700
Term of agreement	44 months
Estimated project costs \| $	$ 4.8
Agreement expiration date	Dec. 31, 2022
Term of prior terminate written notice	4 months

Related-Party Transactions - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Related-party transactions	$ 0
Chief Executive Officer
Related Party Transaction [Line Items]
Proceeds from repayment of loan		$ 200,000
Interest accrued rate		0.60%

Selected Quarterly Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 111	$ 1,801	$ 254	$ 354	$ 111	$ 111	$ 2,111	$ 111	$ 2,520	$ 2,444
Operating expenses	12,819	13,335	15,606	11,990	12,029	12,186	11,568	7,485	53,750	43,268
Loss from operations	(12,708)	(11,534)	(15,352)	(11,636)	(11,918)	(12,075)	(9,457)	(7,374)	(51,230)	(40,824)
Net loss	$ (11,931)	$ (10,748)	$ (14,591)	$ (11,165)	$ (11,697)	$ (11,924)	$ (9,388)	$ (7,368)	$ (48,435)	$ (40,377)
Net loss per share attributable to common stockholders - basic and diluted	$ (0.47)	$ (0.43)	$ (0.59)	$ (0.55)	$ (0.74)	$ (1.66)	$ (4.70)		$ (2.03)	$ (6.00)
Net loss per unit attributable to common unit holders - basic and diluted								$ (4.49)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Subsequent Events - Additional Information (Detail) $ in Millions	Feb. 28, 2019 USD ($)
Subsequent Event \| AbbVie
Subsequent Event [Line Items]
Remaining milestones achieved	$ 2.5