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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2017	Nov. 06, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q3
Trading Symbol	SYBX
Entity Registrant Name	SYNLOGIC, INC.
Entity Central Index Key	0001527599
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		16,285,271

Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 79,175	$ 14,586
Short-term marketable securities	17,397
Prepaid expenses and other current assets	1,656	1,477
Total current assets	98,228	16,063
Property and equipment, net of accumulated depreciation of $2,132 and $810 as of September 30, 2017 and December 31, 2016, respectively	4,911	3,504
Restricted cash	1,097	50
Other assets	234	422
Total assets	104,470	20,039
Current liabilities:
Accounts payable	956	988
Accrued expenses	5,519	2,296
Deferred revenue	444	444
Deferred rent	778	255
Capital lease obligations	264	203
Total current liabilities	7,961	4,186
Long-term liabilities:
Deferred revenue, net of current portion	779	1,112
Deferred rent, net of current portion		1,061
Capital lease obligations, net of current portion	165	177
Total long-term liabilities	944	2,350
Commitments and contingencies
Equity
Preferred Shares, $0.001 par value 5,000,000 shares authorized, none issued and outstanding as of September 30, 2017 and none authorized, issued and outstanding as of December 31, 2016
Common shares, $0.001 par value 250,000,000 and 0 shares authorized as of September 30, 2017 and December 31, 2016. 16,284,885 shares issued and outstanding as of September 30, 2017 and 0 shares issued and outstanding as of December 31, 2016	16
Common units, Issued and outstanding 0 and 1,847,616 units as of September 30, 2017 and December 31, 2016, respectively		592
Additional paid-in capital	155,508
Accumulated other comprehensive income	(2)
Accumulated deficit	(59,957)	(31,248)
Total equity	95,565	8,503
Total liabilities and equity	$ 104,470	20,039
Contingently Redeemable Class A Preferred Units
Long-term liabilities:
Preferred Units, Value		5,000
Equity
Preferred Units, Value		5,000
Class B Preferred Units
Long-term liabilities:
Preferred Units, Value		13,611
Equity
Preferred Units, Value		13,611
Class A Preferred Units
Long-term liabilities:
Preferred Units, Value		25,548
Equity
Preferred Units, Value		$ 25,548

Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Property and equipment, net of accumulated depreciation	$ 2,132	$ 810
Preferred Shares, par value	$ 0.001	$ 0.001
Preferred Shares, Shares Authorized	5,000,000	0
Preferred Shares, Shares Issued	0	0
Preferred Shares, Shares Outstanding	0	0
Common shares, par value	$ 0.001	$ 0.001
Common shares, authorized	250,000,000	0
Common shares, Issued	16,284,885	0
Common shares, outstanding	16,284,885	0
Common units, Issued	0	1,847,616
Common units, outstanding	0	1,847,616
Contingently Redeemable Class A Preferred Units
Preferred Units, Units Issued	0	781,693
Preferred Units, Units Outstanding	0	781,693
Class B Preferred Units
Preferred Units, Units Issued	0	1,029,852
Preferred Units, Units Outstanding	0	1,029,852
Class A Preferred Units
Preferred Units, Units Issued	0	3,922,028
Preferred Units, Units Outstanding	0	3,922,028

Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Revenue	$ 111	$ 111	$ 2,333	$ 333
Operating expenses:
Research and development	8,955	4,133	22,605	9,883
General and administrative	3,231	1,286	8,634	4,555
Total operating expenses	12,186	5,419	31,239	14,438
Loss from operations	(12,075)	(5,308)	(28,906)	(14,105)
Other income (expense):
Interest and investment income	170	3	267	3
Interest expense	(7)	(1)	(22)	(4)
Other expense	(12)		(19)
Other income (expense), net	151	2	226	(1)
Net loss	$ (11,924)	$ (5,306)	$ (28,680)	$ (14,106)
Net loss per share attributable to common shareholders - basic and diluted	$ (1.66)		$ (7.87)
Weighted-average common shares used in computing net loss per share attributable to common shareholders - basic and diluted	7,169,241		3,642,125
Net loss per unit attributable to common unit holders - basic and diluted		$ (3.33)		$ (9.17)
Weighted-average common units used in computing net loss per unit attributable to common unit holders - basic and diluted		1,594,265		1,538,896

Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net loss	$ (11,924)	$ (5,306)	$ (28,680)	$ (14,106)
Other comprehensive loss:
Net unrealized losses on marketable securities	(2)		(2)
Other comprehensive loss	(2)		(2)
Comprehensive loss	$ (11,926)	$ (5,306)	$ (28,682)	$ (14,106)

Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net loss	$ (28,680)	$ (14,106)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	1,324	489
Loss on disposal of assets	5	4
Equity-based compensation expense	1,478	226
Common shares issued for license acquisition	1,750
Accretion/amortization of investment securities	5
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(179)	(695)
Accounts payable and accrued expenses	2,946	571
Deferred revenue	(333)	(333)
Deferred rent	(538)	77
Other assets	188	(67)
Net cash used in operating activities	(22,034)	(13,834)
Cash flows from investing activities:
Net assets acquired in reverse merger, net of transaction costs	40,690
Purchases of marketable securities	(22,855)
Proceeds from maturity of marketable securities	5,450
Changes in restricted cash	(1,047)
Proceeds from sale of property and equipment	11	8
Purchases of property and equipment	(2,531)	(1,746)
Net cash used in investing activities	19,718	(1,738)
Cash flows from financing activities:
Payments on capital lease obligations	(176)	(49)
Proceeds from sale of preferred shares, net of issuance costs	40,433
Proceeds from sale of preferred units, net of issuance costs	26,648	30,938
Net cash provided by financing activities	66,905	30,889
Net increase in cash	64,589	15,317
Cash at beginning of period	14,586	6,179
Cash at end of period	79,175	21,496
Supplemental disclosure of non-cash investing activities:
Landlord funded allowance for tenant improvements		1,295
Transaction costs from reverse merger in accounts payable and accrued expenses	255
Adjustment for property and equipment purchases included in accounts payable and accrued expenses	(9)	(14)
Supplemental disclosure of non-cash financing activities:
Cash paid for interest	22	4
Purchase under capital lease	$ 225
Issuance costs from sale of preferred shares in accounts payable and accrued expenses		$ 100

Nature of Business

9 Months Ended

Sep. 30, 2017

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, which are designed using synthetic biology to genetically reprogram beneficial microbes to treat metabolic and inflammatory diseases and cancer. Synthetic Biotic medicines are generated from Synlogic’ s proprietary drug discovery and development platform. Synlogic applies the principles and tools of synthetic biology to engineer beneficial, probiotic bacteria to perform or deliver critical therapeutic functions, compensating for missing or damaged pathways in patients with these serious diseases. As living medicines, Synthetic Biotic medicines are designed to sense a local disease context within a patient’s body and respond by metabolizing toxic substances or delivering combinations of therapeutic factors.

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 Therapeutic, 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 shareholders of Private Synlogic executed the Synlogic, LLC Contribution Agreement (the “Contribution Agreement”), pursuant to which such common and preferred shareholders contributed such shareholders’ 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 (“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”) (Note 11).

In May 2017, Private Synlogic completed the 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 2017 Plan to replace the canceled incentive units pursuant to the termination of the Synlogic, LLC 2015 Equity Incentive Plan (“2015 LLC Plan”) (Note 10).

On August 28, 2017, Synlogic, Inc., formerly known as Mirna Therapeutics, Inc. (NASDAQ: MIRN) (“Mirna”), completed its business combination with Private Synlogic in accordance with the terms of 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”). On August 25, 2017, in connection with, and prior to the completion of, the Merger, Mirna effected a 1:7 reverse stock split of its common stock (the “Reverse Stock Split”), and on August 28, 2017, immediately after completion of the Merger, Mirna changed its name to “Synlogic, Inc.” (NASDAQ: SYBX) (Note 3).

The Company operates in one operating segment: the discovery and development of Synthetic Biotic medicines. 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. Since incorporation, the Company has devoted substantially all of its efforts to the research and development of its product candidates.

Risks and Uncertainties

At September 30, 2017, the Company had approximately $96.6 million in cash, cash equivalents, and marketable securities, approximately $1.1 million of restricted cash and an accumulated deficit of approximately $60.0 million. Since its inception through September 30, 2017, the Company has primarily financed its operations through the issuance of preferred stock, the AbbVie collaboration, and 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. The Company secured new funding from the sale of Series C convertible preferred stock in May 2017, generating approximately $40.4 million in net proceeds. Additionally, the Company received approximately $40.4 million in net proceeds from the Merger. As a result of the Merger proceeds, the proceeds from the Series C financing in May 2017 and the Series B financing in February 2017, 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

9 Months Ended

Sep. 30, 2017

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

(2)

Summary of Significant Accounting Policies

(a)

Basis of Presentation

The accompanying consolidated financial statements and the related disclosures as of September 30, 2017 and for the three and nine months ended September 30, 2017 and 2016 are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States (“U.S.”) (“U.S. GAAP” or “GAAP”) and the rules and regulations of the Securities Exchange Commission (“SEC”) for interim financial statements. Accordingly, they do not include all of the information and notes required by GAAP for complete financial statements. These interim consolidated financial statements should be read in conjunction with the Company’s 2016 and 2015 audited consolidated financial statements and notes included in the Company’s Current Report on Form 8-K/A filed with the SEC on September 26, 2017. The December 31, 2016 consolidated balance sheet included herein was derived from the audited financial statements as of that date, but does not include all disclosures including notes required by GAAP for complete financial statements. In the opinion of management, the unaudited interim consolidated financial statements reflect all adjustments, consisting of normal and recurring adjustments, necessary for the fair presentation of the Company’s financial position and results of operations for the three and nine months ended September 30, 2017 and 2016. The results of operations for the interim periods are not necessarily indicative of the results to be expected for the year ending December 31, 2017 or any other interim period or future year or period.

(b)

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.

(c)

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, 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.

(d)

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.

(e)

Fair Value

The Company records money market funds at fair value. 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

(f)

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. There were no other-than-temporary impairments for the three or nine months ended September 30, 2017.

(g)

Recently Issued Accounting Pronouncements

The recently issued accounting pronouncements described in the Company’s consolidated financial statements as of and for the year ended December 31, 2016, and the notes thereto included in the Company’s Current Report on Form 8-K/A filed with the SEC on September 26, 2017, have had no material changes during the three and nine months ended September 30, 2017, except as described below.

In May 2014, the FASB issued ASU 2014-09—Revenue from Contracts with Customers (Topic 606), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. This standard is based on the principle that an entity should recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive. This standard also requires additional disclosure about the nature, amount, timing and uncertainty of assets recognized from costs incurred to fulfill a contract. It will be effective for annual reporting periods beginning after December 15, 2017, including interim periods within those annual periods. Early adoption is permitted any time after the original effective date, which for the Company is January 1, 2017. Entities have the option of using either a full retrospective or a modified retrospective approach for the adoption of the new standard. The Company is continuing to assess the impact that this standard will have on its financial statements and the expected method of transition. The Company’s revenue during the nine months ended September 30, 2017 is from its collaboration arrangement. During the fourth quarter of 2017, the Company plans to complete its review to determine the impact that this standard could have on its consolidated financial statements and disclosures.

In March 2016, the FASB issued ASU 2016-09—Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting (“ASU 2016-09”). The amendments in ASU 2016-09 are to simplify several aspects of the accounting for stock-based payment transactions including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. In addition, companies will now have to elect whether to account for forfeitures on share-based payments by (1) recognizing forfeitures of awards as they occur or (2) estimating the number of awards expected to be forfeited and adjusting the estimate when it is likely to change, as is currently required. The Company adopted ASU 2016-09 on April 1, 2017 on a modified retrospective basis, and elected to recognize forfeitures as they occur. The Company recorded an insignificant cumulative effect adjustment as a result of the adoption of this amendment. The adoption did not have a material impact on the Company’s condensed consolidated financial statements and related disclosures.

In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805) Clarifying the Definition of a Business (“ASU 2017-01”), which clarifies the definition of a business with the objective of adding guidance to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The definition of a business affects many areas of accounting including acquisitions, disposals, goodwill, and consolidation. The Company adopted ASU 2017-01 on April 1, 2017 and followed the guidance when determining the accounting treatment of its Merger with Mirna.

Merger with Mirna Therapeutics

9 Months Ended

Sep. 30, 2017

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 shareholders own approximately 83% of the combined company immediately following the closing of the Merger, (ii) Private Synlogic directors hold five of the seven board seats in the combined company, and (iii) Private Synlogic management holds 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 ASU 2017-01. 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(“Merger Exchange Ratio”). The Merger Exchange Ratio was determined through arms’-length negotiations between Mirna and Private Synlogic. Mirna assumed all of the stock options outstanding under the Synlogic 2017 Stock Incentive Plan (“2017 Plan”), with such stock options henceforth representing the right to purchase a number of shares of Mirna’s common stock equal to 0.5532 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 Merger Exchange Ratio for all periods presented.

Immediately after the Merger, there were 16,282,496 shares of the Company’s common stock outstanding. At this time, the former stockholders and optionholders of Private Synlogic owned, or held rights to acquire, approximately 82.4% of the fully-diluted common stock of the Company, which for these purposes is defined as the outstanding common stock of the Company, plus “in the money” options, assuming that all “in the money” options of the Company outstanding immediately prior to the Merger were exercised on a cashless basis immediately prior to the closing of the Merger (the “Fully-Diluted Common Stock of the Company”), with Mirna’s stockholders and optionholders immediately prior to the Merger owning approximately 17.6% of the Fully-Diluted Common Stock of the Company.

On the Merger Closing Date, the Company had approximately 20.9 million shares of common stock outstanding and a market capitalization of approximately $35 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 Company’s 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

9 Months Ended

Sep. 30, 2017

Fair Value Disclosures [Abstract]

Fair Value of Financial Instruments

(4)

Fair Value of Financial Instruments

The table below presents information about the Company’s assets that are measured at fair value on a recurring basis as of September 30, 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.

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


	Fair Value Measurements at Reporting Date Using
	September 30,		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)	$	38,633	$	38,633	$	—	$	—
U.S. government-sponsored securities		400		—		400		—
U.S. Treasury securities		16,997		16,997		—		—

Total	$	56,030	$	55,630	$	400	$	—

Cash equivalents, prepaid expenses and other current assets, accounts payable and accrued expenses at September 30, 2017 and December 31, 2016 are carried at amounts that approximate fair value due to their short-term maturities. Capital lease obligations at September 30, 2017 and December 31, 2016 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

9 Months Ended

Sep. 30, 2017

Investments, Debt and Equity Securities [Abstract]

Available-for-Sale Investments

(5)

Available-for-Sale Investments

The following table summarizes the available-for-sale securities held at September 30, 2017 (in thousands):


September 30, 2017	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair Value
U.S. government-sponsored securities	$	400	$	—	$	—		$	400
U.S. Treasury securities		16,999		—		(2	)		16,997

Total	$	17,399	$	—	$	(2	)	$	17,397

The Company did not have any available-for-sale securities at December 31, 2016.

The contractual maturity of all securities held at September 30, 2017 was one year or less. There were five investments in an unrealized loss position at September 30, 2017, 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 was approximately $17.4 million. 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 September 30, 2017.

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

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Prepaid Expenses and Other Current Assets

9 Months Ended

Sep. 30, 2017

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):


	September 30,		December 31,
	2017		2016
Prepaid insurance	$	539	$	71
Prepaid research and development		644		1,163
Other prepaid		279		212
Other current assets		194		31

	$	1,656	$	1,477

Accrued Expenses

9 Months Ended

Sep. 30, 2017

Payables and Accruals [Abstract]

Accrued Expenses

(7)

Accrued Expenses

Accrued expenses consists of the following (in thousands):


	September 30,		December 31,
	2017		2016
Payroll related	$	1,282	$	1,341
Professional fees		923		522
Research and development		2,874		273
Other		440		160

	$	5,519	$	2,296

Common Stock

9 Months Ended

Sep. 30, 2017

Equity [Abstract]

Common Stock

(8)

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 repurchase options relating to 1,217,040 of these shares, at a price equal to the initial purchase price by the founder, adjusted by the Merger Exchange Ratio. The repurchase option is exercisable should the founder cease providing services to the Company prior to the end of a four-year period beginning in April 2014. As of September 30, 2017, the Company has exercised its repurchase option on 41,819 shares of common stock as services by one of the founders had ceased.

Preferred Stock

9 Months Ended

Sep. 30, 2017

Equity [Abstract]

Preferred Stock

(9)

Preferred Stock

Prior to the Merger, the Company 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.

Equity-based Compensation and Equity Incentive Plans

9 Months Ended

Sep. 30, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Equity-based Compensation and Equity Incentive Plans

(10)

Equity-based Compensation and Equity Incentive Plans

(a)

Equity Compensation

Equity compensation during the three and nine months ended September 30, 2017 and 2016 is derived from restricted stock awards and stock options issued under the 2017 Plan, the Synlogic, Inc. 2015 Stock Incentive Plan (“2015 Plan”), from incentive units issued under the 2015 LLC Plan and from a restricted common unit grant. The Company has recorded total equity-based compensation expense of approximately $0.7 million and $1.5 million for the three and nine months ended September 30, 2017, respectively, and approximately $0.1 million and $0.2 million for the three and nine months ended September 30, 2016, respectively, which is based on the number of awards ultimately expected to vest.

The Company is displaying all equity associated with the 2017 Plan in its post-Merger amounts, as impacted by the exchange ratio.

The following table summarizes equity-based compensation expense within the Company’s consolidated statements of operations and comprehensive loss for the three and nine months ended September 30, 2017 and 2016 (in thousands):


	Three months ended September 30,				Nine Months ended September 30,
	2017		2016		2017		2016
Research and development	$	380	$	29	$	883	$	83
General and administrative		301		44		595		143

	$	681	$	73	$	1,478	$	226

The following table summarizes equity-based compensation expense by type of award for the three and nine months ended September 30, 2017 and 2016 (in thousands):


	Three months ended September 30,				Nine months ended September 30,
	2017		2016		2017		2016
Stock options	$	508	$	—	$	992	$	—
Restricted stock awards		173		—		298		—
Incentive units		—	$	39		132		125
Restricted common units		—		34		56		101

	$	681	$	73	$	1,478	$	226

(b)

Awards Issued Under the Synlogic, LLC 2015 Equity Incentive Plan

(i)

Incentive Units

In October 2015, Private Synlogic’s Board of Directors adopted the 2015 LLC Plan, which provided for the grant of equity incentive units to employees, officers, directors or consultants. The awards 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. 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 Private Synlogic. 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.

Private Synlogic measured and recorded the value of incentive units granted to non-employees 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 three and nine months ended September 30, 2017 and 122,536 and 255,672 incentive units were issued during the three and nine months ended September 30, 2016, respectively. 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 unrecognized compensation expense related to incentive units as of September 30, 2017.

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 September 30, 2017		—		$	—	$	—	$	—

Vested or expected to vest at September 30, 2017		—		$	—	$	—	$	—

(ii)

Restricted Common Units

No restricted common unit awards were issued during the three and nine months ended September 30, 2017 and 2016. During the three and nine months ended September 30, 2017, 0 and 68,280 units, respectively, vested and approximately $0 and $0.1 million, respectively, in equity compensation was recognized. During the three and nine months ended September 30, 2016, 40,968 units and 218,497 units vested, respectively, and approximately $34,000 and approximately $0.1 million, respectively, in equity-based compensation was recognized. In May 2017, the restricted common unit award was 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 September 30, 2017.

(c)

Awards Issued Under the Synlogic, Inc. 2017 Stock Incentive Plan and Synlogic, Inc. 2015 Stock Incentive Plan

In May 2017, Private Synlogic adopted the 2017 Plan which provided for the grant of incentive stock options, non-qualified stock options, restricted and unrestricted stock awards and other stock-based awards. Pursuant to the 2017 Reorganization, Private Synlogic issued restricted common stock awards under the 2017 Stock Incentive Plan to replace the canceled incentive units pursuant to the termination of the 2015 LLC Plan. In certain instances, Private Synlogic also issued stock options related to the cancelled incentive units. Pursuant to the Merger Agreement, each option to purchase shares of Private Synlogic common stock under the 2017 Plan that was outstanding and unexercised immediately prior to the Merger was converted into and became an option to purchase shares of the Company’s common stock based on the Merger Exchange Ratio of 0.5532 and the Company assumed the 2017 Plan.

The 2015 Equity Incentive Award Plan (“2015 Plan”) was adopted by the Company in 2015 and remained active after the Merger, and now functions as an active plan for the Company. 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.

(i)

Stock Options

During the three and nine months ended September 30, 2017, 194,260 and 843,965 stock options, respectively, were granted to employees and consultants. There were no stock options granted during the three and nine months ended September 30, 2016.

The weighted average assumptions used in the Black-Scholes option-pricing model for awards issued under the 2015 Plan and the 2017 Plan during both the three and nine months ended September 30, 2017 were:


	Three months ended September 30, 2017						Nine months ended September 30, 2017
	Employee			Nonemployee			Employee			Nonemployee
Expected term		6.2 years			0.2 - 1.6 years			6.1 years			0.2 - 1.6 years
Weighted-average, risk-free interest rate		2.1	%		1.0	%		2.0	%		1.1	%
Expected volatility		70.2	%		63.9	%		70.1	%		65.3	%
Dividend yield		—			—			—			—

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


	Stock options outstanding
						Weighted
				Weighted		average		Aggregate
				average		remaining		Intrinsic
	Number of			exercise		contractual		value
	options			price		term		(in thousands)
Outstanding at December 31, 2016		—		$	—		—	$	—
Options granted upon 2017 Reorganization		295,289			13.53		5.7		1,621
Granted		548,676			15.25		6.2		2,069
Exercised		—			—		—		—
Forfeited		(2,455	)		13.53		6.1		(14	)

Outstanding at September 30, 2017		841,510						$	3,676

Vested or expected to vest at September 30, 2017		841,510			14.65		6.0	$	3,676

Exercisable at September 30, 2017		109,745			13.57		5.4	$	598

During the three and nine months ended September 30, 2017, approximately $0.5 million and $1.0 million in equity compensation was recognized related to stock options related to employees, respectively.

The weighted average grant date fair value per share of options granted to employees during the three and nine months ended September 30, 2017 was approximately $11.89 and $12.58, respectively. The grant date fair value of the options awarded to employees during the three and nine months ended September 30, 2017 was approximately $2.3 million and $7.5 million, respectively. No options were exercised during the three and nine months ended September 30, 2017.

As of September 30, 2017, there was approximately $6.8 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 6.2 years. The total unrecognized share-based compensation cost will be adjusted for actual forfeitures as they occur. In addition, there was approximately $0.2 million of unrecognized share-based compensation, related to unvested stock option grants to non-employees which is expected to be recognized over a weighted average period of 0.6 years. The amount of equity-based compensation expense related to non-employees that will ultimately be recorded will depend on the remeasurement of the outstanding awards through their vesting date.

(ii)

Restricted Common Stock

During the three and nine months ended September 30, 2017, 2,884 and 1,062,795 shares of common stock, respectively were granted. As part of the 2017 Reorganization in May 2017, 1,059,911 shares of restricted common stock (adjusted for the merger exchange) were granted in exchange for the restricted common units that were cancelled as part of the 2017 Reorganization. These shares retained the same vesting schedule as the cancelled 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. No restricted common stock was granted during the three and nine months ended September 30, 2016.

The following table shows restricted stock activity:


	Restricted stock awards
				Grant date
	Number of			fair value
	shares			(per share)
Unvested at December 31, 2016		—		$	—
Awards exchanged upon 2017 Reorganization		1,059,911			13.35
Granted		2,884			19.01
Vested		(604,505	)		9.38
Forfeited		(3,457	)		13.53

Unvested at September 30, 2017		454,833		$	13.56

During the three months ended September 30, 2017, 88,840 shares of restricted stock vested and approximately $0.2 million in equity compensation was recognized. During the nine months ended September 30, 2017, 604,505 shares of restricted stock vested, of which 501,231 shares were vested at the time of grant and 103,274 shares represent continued vesting of the grants and $0.3 million in equity compensation was recognized. During both the three and nine months ended September 30, 2016, no restricted stock vested and no equity-based compensation was recognized associated with restricted stock awards.

As of September 30, 2017, there was approximately $0.7 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 2.4 years. The total unrecognized share-based compensation cost will be adjusted for actual forfeitures as they occur. In addition, there was approximately $0.3 million of unrecognized share-based compensation, related to unvested restricted stock awards granted to non-employees which is expected to be recognized over a weighted average period of 0.7 years.

Significant Agreements

9 Months Ended

Sep. 30, 2017

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Significant Agreements

(11)

Significant Agreements

(a)

AbbVie Collaboration Agreement

In July 2015, the Company entered into an Agreement and Plan of Merger (“the Agreement”) with AbbVie under which the Company granted an exclusive option to AbbVie to purchase IBDCo and agreed to collaborate in researching and developing an Investigatory New Drug (“IND”) candidate for the treatment of IBD.

In exchange for the exclusive option to acquire IBDCo, initial research and development services, ongoing patent defense, and participation on the joint steering committee (“JSC”), 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 development milestone payments, all of which were considered substantive, as well as an option exercise fee upon the execution of their option to buy IBDCo. In May 2017, the Company achieved the first development milestone under the Agreement for consideration of $2.0 million. The agreement also provides for royalty payments and payments upon the achievement of certain clinical, regulatory and commercial milestones.

The Agreement sets forth the Company’s and AbbVie’s respective obligations for development and delivery of an IND candidate package using reasonable commercial efforts. The JSC will make a determination as to the continuation of the collaboration at the achievement of the milestones.

At the inception of the Agreement, the Company identified the following deliverables: (i) an exclusive option to purchase IBDCo, (ii) research and development services and ongoing patent defense, and (iii) participation on the JSC. The Company also identified contingent deliverables related to four research and development milestones, delivery of an IND candidate package milestone, and transfer of ownership of IBDCo upon exercise of the option to buy IBDCo. The contingent deliverables have been excluded from the initial allocation and will be treated as a separate unit of accounting when and if delivered.

The Company concluded that none of the three deliverables identified at the inception of the Agreement has stand-alone value from the other undelivered elements. Accordingly, these deliverables represent a single unit of accounting.

As of September 30, 2017, the only consideration that is fixed and determinable is the nonrefundable upfront payment of $2.0 million. The consideration relates to the three identified deliverables that comprise the single unit of accounting, which will be recognized over the period of performance. The period of performance will be through the option period, which is closely tied to the completion of the research and development collaboration with AbbVie, and has been estimated to be 54 months. The Company will periodically review and, if necessary, revise the estimated period of performance.

During the three and nine months ended September 30, 2017, the Company recognized approximately $0.1 million and approximately $2.3 million, respectively, in revenue associated with the Agreement. During the three and nine months ended September 30, 2016, the Company recognized approximately $0.1 million and approximately $0.3 million, respectively, in revenue associated with the Agreement. As of September 30, 2017, there was approximately $1.2 million of deferred revenue related to the Agreement, which is classified as current or noncurrent in the consolidated balance sheets based on the Company’s estimate of revenue that will be recognized within the next twelve months. All costs associated with the collaboration agreement will be recorded in research and development expense in the consolidated statements of operations and comprehensive loss in the period incurred.

(b)

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. 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, as the licenses do not have future alternative use, in accordance with ASC Topic 730, Research and Development.

Net Loss per Share/Unit

9 Months Ended

Sep. 30, 2017

Earnings Per Share [Abstract]

Net Loss per Share/Unit

(12)

Net Loss per Share/Unit

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

The Company computed basic and diluted net loss per share/unit using the two-class method, which gives effect to the impact of outstanding participating securities. As the three and nine months ended September 30, 2017 and 2016 resulted in net losses attributable to common shareholders/unit holders, 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/unit because the preferred shareholders/unit holders do not participate in losses of the Company. Accordingly, for periods in which the Company reports a net loss attributable to common shareholders/unit holders, diluted net loss per share/unit attributable to common shareholders/unit holders is the same as basic net loss per share/unit attributable to common shareholders/unit holders, since dilutive common shares/units 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 shares 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, on the Merger Closing Date, the Company issued shares of its common stock to Private Synlogic shareholders, at the Merger 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 Merger Exchange Ratio was calculated by a formula pursuant to the Merger Agreement. For the purposes of calculating net loss per share, the Merger Exchange Ratio was applied retroactively to all periods presented.

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


	Three months ended September 30,						Nine months ended September 30,
	2017			2016			2017			2016
Numerator:
Net loss attributable to common shareholders	$	(11,924	)				$	(28,680	)	$	—

Denominator:
Weighted-average common shares outstanding - basic and diluted		7,169,241			—			3,642,125			—

Net loss per share attributable to common shareholders - basic and diluted	$	(1.66	)	$	—		$	(7.87	)	$	—

Numerator:
Net loss attributable to common unit holders	$	—		$	(5,306	)	$	—		$	(14,106	)

Denominator:
Weighted-average common units outstanding - basic and diluted		—			1,594,265			—			1,538,896

Net loss per unit attributable to common unit holders - basic and diluted	$	—		$	(3.33	)	$	—		$	(9.17	)

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

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


	As of September 30,
	2017		2016
Unvested restricted common unit awards		—		436,977
Unvested restricted common stock awards		454,833		—
Outstanding options to purchase common stock		841,510		—

Commitments and Contingencies

9 Months Ended

Sep. 30, 2017

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

(13)

Commitments and Contingencies

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.

In July 2017, the Company entered into an agreement to lease approximately 41,346 square feet of laboratory and office space in Cambridge, Massachusetts. Annual rent is approximately $3.1 million. The ten-year lease is estimated to commence in January 2018 and contains provisions for a free-rent period, annual rent increases and an allowance for tenant improvements. Additionally, the Company has committed to a tenant improvement investment of approximately $1.6 million. 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 2017, the Company entered into an agreement to terminate its existing lease of laboratory and office space in Cambridge, Massachusetts at a date that is 30 days after the commencement of its new lease. No penalties are associated with the termination of the lease. 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 our leasehold improvements associated with the 200 Sidney Street facility from 63 months to seven months.

Related-Party Transactions

9 Months Ended

Sep. 30, 2017

Related-Party Transactions

(14)

Related-Party Transactions

During the nine months ended September 30, 2017, the Company received repayment of the loan to its 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%.

The Company contracted services from one of its principal investors for the Company’s former chief medical officer who was employed by the principal investor, as well as employed to support separate portfolio companies of the investor. The Company made no payments during the three and nine months ended September 30, 2017 and paid approximately $39,000 and $136,000 related to reimbursement for a portion of the salary of the former chief medical officer for the three and nine months ended September 30, 2016, respectively.

The Company contracted the services of The Orphan Group which specializes in supporting biotechnology companies in developing therapeutics toward diseases of high unmet medical needs in rare disorders. The Orphan Group is owned by the Company’s former chief operating officer. The Company made no payments to the Orphan Group during the three and nine months ended September 30, 2017 and paid $0 and approximately $13,000 for contracted services during the three and nine months ended September 30, 2016, respectively.

Subsequent Events

9 Months Ended

Sep. 30, 2017

Subsequent Events [Abstract]

Subsequent Events

(15)

Subsequent Events

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, par value $0.001 per share, 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.

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2017

Accounting Policies [Abstract]

Basis of Presentation

(a)

Basis of Presentation

Principles of Consolidation

(b)

Principles of Consolidation

Use of Estimates

(c)

Use of Estimates

Concentration of Credit Risk

(d)

Concentration of Credit Risk

Fair Value

(e)

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

(f)

Available-for-Sale Securities

Recently Issued Accounting Pronouncements

(g)

Recently Issued Accounting Pronouncements

- Definition

Disclosure of accounting policy for investments in debt and equity securities that are classified as available-for-sale. This policy also may describe the entity's accounting treatment for transfers between investment categories, how the entity determines whether impairments of available-for-sale securities are other than temporary, and how the fair values of such securities are determined.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84158767&loc=d3e18780-107790

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 5.M)
-URI http://asc.fasb.org/extlink&oid=65877573&loc=d3e87990-122713

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Merger with Mirna Therapeutics (Tables)

9 Months Ended

Sep. 30, 2017

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)

9 Months Ended

Sep. 30, 2017

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
	September 30,		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)	$	38,633	$	38,633	$	—	$	—
U.S. government-sponsored securities		400		—		400		—
U.S. Treasury securities		16,997		16,997		—		—

Total	$	56,030	$	55,630	$	400	$	—

Available-for-Sale Investments (Tables)

9 Months Ended

Sep. 30, 2017

Investments, Debt and Equity Securities [Abstract]

Summary of Available-for-Sale Securities Held

The following table summarizes the available-for-sale securities held at September 30, 2017 (in thousands):


September 30, 2017	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair Value
U.S. government-sponsored securities	$	400	$	—	$	—		$	400
U.S. Treasury securities		16,999		—		(2	)		16,997

Total	$	17,399	$	—	$	(2	)	$	17,397

Prepaid Expenses and Other Current Assets (Tables)

9 Months Ended

Sep. 30, 2017

Text Block [Abstract]

Schedule of Prepaid Expenses and Other Current Assets

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


	September 30,		December 31,
	2017		2016
Prepaid insurance	$	539	$	71
Prepaid research and development		644		1,163
Other prepaid		279		212
Other current assets		194		31

	$	1,656	$	1,477

Accrued Expenses (Tables)

9 Months Ended

Sep. 30, 2017

Payables and Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses consists of the following (in thousands):


	September 30,		December 31,
	2017		2016
Payroll related	$	1,282	$	1,341
Professional fees		923		522
Research and development		2,874		273
Other		440		160

	$	5,519	$	2,296

Equity-based Compensation and Equity Incentive Plans (Tables)

9 Months Ended

Sep. 30, 2017

Schedule of Equity-based Compensation Expenses


	Three months ended September 30,				Nine Months ended September 30,
	2017		2016		2017		2016
Research and development	$	380	$	29	$	883	$	83
General and administrative		301		44		595		143

	$	681	$	73	$	1,478	$	226

Schedule of Equity-based Compensation Expenses by Award Type

The following table summarizes equity-based compensation expense by type of award for the three and nine months ended September 30, 2017 and 2016 (in thousands):


	Three months ended September 30,				Nine months ended September 30,
	2017		2016		2017		2016
Stock options	$	508	$	—	$	992	$	—
Restricted stock awards		173		—		298		—
Incentive units		—	$	39		132		125
Restricted common units		—		34		56		101

	$	681	$	73	$	1,478	$	226

Schedule of Restricted Stock Activity

The following table shows restricted stock activity:


	Restricted stock awards
				Grant date
	Number of			fair value
	shares			(per share)
Unvested at December 31, 2016		—		$	—
Awards exchanged upon 2017 Reorganization		1,059,911			13.35
Granted		2,884			19.01
Vested		(604,505	)		9.38
Forfeited		(3,457	)		13.53

Unvested at September 30, 2017		454,833		$	13.56

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 September 30, 2017		—		$	—	$	—	$	—

Vested or expected to vest at September 30, 2017		—		$	—	$	—	$	—

2015 and 2017 Plan

Schedule of Stock Option Activity

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


	Stock options outstanding
						Weighted
				Weighted		average		Aggregate
				average		remaining		Intrinsic
	Number of			exercise		contractual		value
	options			price		term		(in thousands)
Outstanding at December 31, 2016		—		$	—		—	$	—
Options granted upon 2017 Reorganization		295,289			13.53		5.7		1,621
Granted		548,676			15.25		6.2		2,069
Exercised		—			—		—		—
Forfeited		(2,455	)		13.53		6.1		(14	)

Outstanding at September 30, 2017		841,510						$	3,676

Vested or expected to vest at September 30, 2017		841,510			14.65		6.0	$	3,676

Exercisable at September 30, 2017		109,745			13.57		5.4	$	598

Schedule of Weighted Average Assumption Used Black-Scholes Option-pricing Model for Awards


	Three months ended September 30, 2017						Nine months ended September 30, 2017
	Employee			Nonemployee			Employee			Nonemployee
Expected term		6.2 years			0.2 - 1.6 years			6.1 years			0.2 - 1.6 years
Weighted-average, risk-free interest rate		2.1	%		1.0	%		2.0	%		1.1	%
Expected volatility		70.2	%		63.9	%		70.1	%		65.3	%
Dividend yield		—			—			—			—

Net Loss per Share/Unit (Tables)

9 Months Ended

Sep. 30, 2017

Earnings Per Share [Abstract]

Schedule of Computation of Basic and Diluted Net Loss per Share


	Three months ended September 30,						Nine months ended September 30,
	2017			2016			2017			2016
Numerator:
Net loss attributable to common shareholders	$	(11,924	)				$	(28,680	)	$	—

Denominator:
Weighted-average common shares outstanding - basic and diluted		7,169,241			—			3,642,125			—

Net loss per share attributable to common shareholders - basic and diluted	$	(1.66	)	$	—		$	(7.87	)	$	—

Numerator:
Net loss attributable to common unit holders	$	—		$	(5,306	)	$	—		$	(14,106	)

Denominator:
Weighted-average common units outstanding - basic and diluted		—			1,594,265			—			1,538,896

Net loss per unit attributable to common unit holders - basic and diluted	$	—		$	(3.33	)	$	—		$	(9.17	)

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


	As of September 30,
	2017		2016
Unvested restricted common unit awards		—		436,977
Unvested restricted common stock awards		454,833		—
Outstanding options to purchase common stock		841,510		—

Nature of Business - Additional Information (Detail) $ in Thousands		1 Months Ended	9 Months Ended
	Aug. 25, 2017	May 31, 2017 USD ($)	Sep. 30, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)
Cash and Cash Equivalents [Line Items]
Reserve stock split of common stock prior to its merger	0.14
Number of operating segment \| Segment			1
Cash, cash equivalents, and marketable securities			$ 96,600
Restricted cash			1,097	$ 50
Accumulated deficit			$ (59,957)	$ (31,248)
Series C Preferred Stock
Cash and Cash Equivalents [Line Items]
Proceeds from sale of preferred stock		$ 40,400
Mirna therapeutics
Cash and Cash Equivalents [Line Items]
Net proceeds from divestiture of businesses and interests in affiliates, total		$ 40,400

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
Accounting Policies [Abstract]
Other than Temporary Impairment	$ 0	$ 0

Merger with Mirna Therapeutics - Additional Information (Detail) $ in Millions	Aug. 28, 2017 USD ($) Director shares	Sep. 30, 2017 shares	Dec. 31, 2016 shares
Business Acquisition [Line Items]
Number of members on Board of Directors \| Director	7
Exchange ratio of common stock	0.5532
Common stock outstanding \| shares	16,282,496	16,284,885	0
Percentage of diluted common stock	82.40%
Mirna therapeutics
Business Acquisition [Line Items]
Common stock outstanding \| shares	20,900,000
Percentage of diluted common stock	17.60%
Market capitalization \| $	$ 35.0
Fair value, net asset (Liability) \| $	$ 42.6
Synlogic
Business Acquisition [Line Items]
Number of members on Board of Directors \| Director	5
Merger Agreement \| 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
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) $ in Thousands	Sep. 30, 2017 USD ($)
Fair Value Asset Measured On Recurring Basis [Line Items]
Marketable securities, current	$ 17,397
Recurring
Fair Value Asset Measured On Recurring Basis [Line Items]
Total assets	56,030
Recurring \| U.S. government-sponsored securities
Fair Value Asset Measured On Recurring Basis [Line Items]
Marketable securities, current	400
Recurring \| U.S. Treasury securities
Fair Value Asset Measured On Recurring Basis [Line Items]
Marketable securities, current	16,997
Recurring \| Money market funds
Fair Value Asset Measured On Recurring Basis [Line Items]
Cash and Cash Equivalents	38,633
Recurring \| Quoted Prices in Active Markets for Idential Assets (Level 1)
Fair Value Asset Measured On Recurring Basis [Line Items]
Total assets	55,630
Recurring \| Quoted Prices in Active Markets for Idential Assets (Level 1) \| U.S. Treasury securities
Fair Value Asset Measured On Recurring Basis [Line Items]
Marketable securities, current	16,997
Recurring \| Quoted Prices in Active Markets for Idential Assets (Level 1) \| Money market funds
Fair Value Asset Measured On Recurring Basis [Line Items]
Cash and Cash Equivalents	38,633
Recurring \| Significant Other Observable Inputs (Level 2)
Fair Value Asset Measured On Recurring Basis [Line Items]
Total assets	400
Recurring \| Significant Other Observable Inputs (Level 2) \| U.S. government-sponsored securities
Fair Value Asset Measured On Recurring Basis [Line Items]
Marketable securities, current	400
Recurring \| Significant Unobservable Inputs (Level 3)
Fair Value Asset Measured On Recurring Basis [Line Items]
Total assets	0
Recurring \| Significant Unobservable Inputs (Level 3) \| U.S. government-sponsored securities
Fair Value Asset Measured On Recurring Basis [Line Items]
Marketable securities, current	0
Recurring \| Significant Unobservable Inputs (Level 3) \| U.S. Treasury securities
Fair Value Asset Measured On Recurring Basis [Line Items]
Marketable securities, current	0
Recurring \| Significant Unobservable Inputs (Level 3) \| Money market funds
Fair Value Asset Measured On Recurring Basis [Line Items]
Cash and Cash Equivalents	$ 0

Available-for-Sale Securities - Summary of Available-for-Sale Securities Held (Detail) $ in Thousands	Sep. 30, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 17,399
Gross unrealized gains	0
Gross unrealized losses	(2)
Fair Value	17,397
U.S. government-sponsored securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	400
Gross unrealized gains	0
Fair Value	400
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	16,999
Gross unrealized gains	0
Gross unrealized losses	(2)
Fair Value	$ 16,997

Available-for-Sale Securities - Additional Information (Detail) $ in Thousands	Sep. 30, 2017 USD ($) Investment
Investments, Debt and Equity Securities [Abstract]
Number of investments in unrealized loss position, less than twelve months	5
Number of investments in unrealized loss position, more than twelve months	0
Aggregate fair value of securities in unrealized loss position \| $	$ 17,397

Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid insurance	$ 539	$ 71
Prepaid research and development	644	1,163
Other prepaid	279	212
Other current assets	194	31
Total	$ 1,656	$ 1,477

Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Payroll related	$ 1,282	$ 1,341
Professional fees	923	522
Research and development	2,874	273
Other	440	160
Total accrued expenses	$ 5,519	$ 2,296

Common Stock - Additional Information (Detail)	9 Months Ended	118 Months Ended
Common Stock - Additional Information (Detail)	Sep. 30, 2017 Vote shares	Sep. 30, 2017 USD ($) shares
Stockholders' Equity Note [Abstract]
Number of votes per common stock \| Vote	1
Cash dividends \| $		$ 0
Repurchase options share	1,217,040	1,217,040
Repurchase options share exercised	41,819
Repurchase option exercisable period	4 years

Preferred Stock - Additional Information (Detail)	Aug. 28, 2017
Preferred Stock [Abstract]
Exchange ratio of common stock	0.5532

Equity-based Compensation and Equity Incentive Plans - Additional Information (Detail)		1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2015	May 31, 2017 shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) shares	Aug. 28, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense \| $			$ 681,000	$ 73,000	$ 1,478,000	$ 226,000
Exchange ratio of common stock							0.5532
Weighted average grant date fair value per share of options granted to employees \| $ / shares			$ 11.89		$ 12.58
The grant date fair value of the options awarded to employees \| $			$ 2,300,000		$ 7,500,000
Option excercised			0		0
Employees and Consultants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted			194,260	0	843,965	0
Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense \| $			$ 6,800,000		$ 6,800,000
Unrecognized compensation cost, period of recognition					6 years 2 months 12 days
Nonemployee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense \| $			200,000		$ 200,000
Unrecognized compensation cost, period of recognition					7 months 6 days
2015 and 2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense \| $			700,000	$ 100,000	$ 1,500,000	$ 200,000
Granted					548,676
Option excercised					0
2015 and 2017 Plan \| 2017 Reorganization
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted					295,289
Plan 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options awards, vesting percentage	25.00%
Options vested period	36 months
Award vesting description					The awards generally vested 25% after one year and ratably monthly thereafter over the next 36 months.
Restricted Stock Awards \| Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense \| $			$ 200,000	$ 0	$ 26,000	$ 0
Vested			88,840	0	604,505	0
Granted			2,884	0	1,062,795	0
Share vasted at the time of grant					501,231
Restricted Stock Vested Shares			103,274		103,274
Restricted Stock Awards \| Employee \| Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense \| $			$ 700,000		$ 700,000
Unrecognized compensation cost, period of recognition					2 years 4 months 24 days
Restricted Stock Awards \| Nonemployee \| Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense \| $			$ 300,000		$ 300,000
Unrecognized compensation cost, period of recognition					8 months 12 days
Restricted Stock Awards \| 2017 Reorganization \| Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted		1,059,911			1,059,911
Incentive Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense \| $				$ 39,000	$ 132,000	$ 125,000
Units issued			0	122,536	0	255,672
Unrecognized compensation expense \| $			$ 0		$ 0
Incentive Units \| Plan 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted					0
Weighted average grant date fair value per share of options granted to employees \| $ / shares					$ 0
Restricted Stock Units \| Common Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense \| $			$ 0	$ 34,000	$ 56,000	$ 101,000
Units issued			0	0	0	0
Vested			0	40,968	68,280	218,497
Unrecognized compensation expense \| $			$ 0		$ 0

Equity-based Compensation and Equity Incentive Plans - Schedule of Equity-based Compensation Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense	$ 681	$ 73	$ 1,478	$ 226
Research and Development Expense
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense	380	29	883	83
General and Administrative Expense
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense	$ 301	$ 44	$ 595	$ 143

Equity-based Compensation and Equity Incentive Plans - Schedule of Equity-based Compensation Expenses by Award Type (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense	$ 681	$ 73	$ 1,478	$ 226
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense	508		992
Restricted Stock Awards \| Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense	173		298
Incentive Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense		39	132	125
Restricted Stock Units \| Common Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense	$ 0	$ 34	$ 56	$ 101

Equity-based Compensation and Equity Incentive Plans - Schedule of Stock Option Activity Under 2015 LLC Plan (Detail)	3 Months Ended	9 Months Ended
	Sep. 30, 2017 $ / shares shares	Sep. 30, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Weighted-average grant date fair value	$ 11.89	$ 12.58
Plan 2015 \| Incentive Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, Number of Non-vested Units \| shares		971,906
Granted, Number of Units \| shares		0
Vested, Number of Units \| shares		(73,719)
Forfeited, Number of Units \| shares		(260,145)
Ending balance, Number of Non-vested Units \| shares	0	0
Vested or expected to vest, Number of Units \| shares	0	0
Beginning balance, Weighted-average price		$ 5.22
Granted, Weighted-average price		0
Vested, Weighted-average price		4.01
Forfeited, Weighted-average price		4.19
Ending balance, Weighted-average price	$ 0	0
Weighted-average price, Vested or expected to vest at September 30, 2017	0	0
Beginning Balance, Weighted-average grant date fair value		1.01
Granted, Weighted-average grant date fair value		0
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	0
Weighted-average grant date fair value, Vested or expected to vest at September 30, 2017	0	0
Plan 2015 \| Incentive Units \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, Weighted-average price		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	0
Weighted-average price, Vested or expected to vest at September 30, 2017	$ 0	$ 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 \| shares		(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 Weighted Average Assumption Used Black-Scholes Option-pricing Model for Awards (Detail) - 2015 and 2017 Plan	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	6 years 2 months 13 days	6 years 1 month 6 days
Weighted-average, risk-free interest rate	2.10%	2.00%
Expected volatility	70.20%	70.10%
Dividend yield	0.00%	0.00%
Nonemployee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average, risk-free interest rate	1.00%	1.10%
Expected volatility	63.90%	65.30%
Dividend yield	0.00%	0.00%
Minimum \| Nonemployee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	2 months 12 days	2 months 12 days
Dividend yield	0.00%	0.00%
Maximum \| Nonemployee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	1 year 7 months 6 days	1 year 7 months 6 days
Dividend yield	0.00%	0.00%

Equity-based Compensation and Equity Incentive Plans - Schedule of Stock Option Activity Under 2015 and 2017 Plan (Detail) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercised, Number of options \| shares	0	0
2015 and 2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Number of options \| shares		548,676
Exercised, Number of options \| shares		0
Forfeited, Number of options \| shares		(2,455)
Outstanding Ending balance, Number of options \| shares	841,510	841,510
Number of options, Vested or expected to vest at September 30, 2017 \| shares	841,510	841,510
Number of options, Exercisable at September 30, 2017 \| shares	109,745	109,745
Beginning balance, Weighted-average price \| $ / shares		$ 0
Granted, Weighted-average price \| $ / shares		15.25
Exercised, Weighted-average price \| $ / shares		0
Forfeited, Weighted-average price \| $ / shares		13.53
Ending balance, Weighted-average price \| $ / shares	$ 0	0
Weighted-average price Vested or expected to vest at September 30, 2017 \| $ / shares	14.65	14.65
Weighted-average price, Exercisable at September 30, 2017 \| $ / shares	$ 13.57	$ 13.57
Granted, Weighted average remaining contractual term		6 years 2 months 13 days
Forfeited, Weighted average remaining contractual term		6 years 1 month 6 days
Weighted average remaining contractual term, Vested or expected to vest at September 30, 2017		6 years
Weighted average remaining contractual term, Exercisable at September 30, 2017		5 years 4 months 24 days
Ending balance, Aggregate Intrinsic value \| $	$ 3,676	$ 3,676
Aggregate Intrinsic value, Vested or expected to vest at September 30, 2017 \| $	3,676	3,676
Aggregate Intrinsic value, Exercisable at September 30, 2017 \| $	$ 598	$ 598
2015 and 2017 Plan \| 2017 Reorganization
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Number of options \| shares		295,289
Granted, Weighted-average price \| $ / shares		$ 13.53
Options granted upon 2017 Reorganization, Weighted average remaining contractual term		5 years 8 months 12 days
Options granted upon 2017 Reorganization, Aggregate Intrinsic value \| $		$ 1,621
Granted, Aggregate Intrinsic value \| $		2,069
Forfeited, Aggregate Intrinsic value \| $		$ (14)

Equity-based Compensation and Equity Incentive Plans - Schedule of Restricted Stock Activity (Detail) - Restricted Stock Awards - Common Stock - $ / shares	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Granted, Number of shares		2,884	0	1,062,795	0
Vested, Number of shares		(88,840)	0	(604,505)	0
Forfeited, Number of shares				(3,457)
Ending balance, Number of unvested shares		454,833		454,833
Vested, Grant date fair value				$ 9.38
Forfeited, Grant date fair value				13.53
Ending balance, Unvested Grant date fair value		$ 13.56		$ 13.56
2017 Reorganization
Granted, Number of shares	1,059,911			1,059,911
Granted, Grant date fair value				$ 13.35
2017 Plan
Granted, Number of shares				2,884
Granted, Grant date fair value				$ 19.01

Significant Agreements - Additional Information (Detail)	1 Months Ended			3 Months Ended		9 Months Ended
	May 31, 2017 USD ($)	Apr. 30, 2017 USD ($) shares	Dec. 31, 2015 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Milestone	Sep. 30, 2016 USD ($)
Acquisition Date [Line Items]
Recognition of revenue				$ 111,000	$ 111,000	$ 2,333,000	$ 333,000
AbbVie
Acquisition Date [Line Items]
Recognition of revenue				100,000	$ 100,000	2,300,000	$ 300,000
Deferred revenue, net of current portion				$ 1,200,000		1,200,000
AbbVie \| IBDCo
Acquisition Date [Line Items]
Upfront non refundable payment			$ 2,000,000
Potential development milestone payment						$ 16,500,000
Development milestone payment	$ 2,000,000
Number of research and development milestones \| Milestone						4
Collaboration in research and develpoment						54 months
Massachusetts Institute of Technology and Boston University License Agreement
Acquisition Date [Line Items]
Recognition of license fees		$ 1,800,000
Massachusetts Institute of Technology and Boston University License Agreement \| Common Units
Acquisition Date [Line Items]
Common units issued for license \| shares		325,377
Massachusetts Institute of Technology and Boston University License Agreement \| Common Stock \| 2017 Reorganization
Acquisition Date [Line Items]
Common stock, converted \| shares		325,377
Massachusetts Institute of Technology and Boston University License Agreement \| Mirna therapeutics \| Common Stock
Acquisition Date [Line Items]
Common stock, converted \| shares		179,999
Massachusetts Institute of Technology and Boston University License Agreement \| Intellectual Property
Acquisition Date [Line Items]
License acquired		$ 50,000
Massachusetts Institute of Technology and Boston University License Agreement \| Patents
Acquisition Date [Line Items]
Payment for prior patent costs		$ 300,000

Net Loss per Share/Unit - Additional Information (Detail)	Aug. 28, 2017
Earnings Per Share [Abstract]
Exchange ratio of common stock	0.5532

Net Loss per Share/Unit - Schedule of Computation of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net loss attributable to common shareholders	$ (11,924)	$ (5,306)	$ (28,680)	$ (14,106)
Denominator:
Weighted-average common shares outstanding - basic and diluted	7,169,241		3,642,125
Net loss per share attributable to common shareholders - basic and diluted	$ (1.66)		$ (7.87)
Numerator:
Net loss attributable to common unit holders		$ (5,306)		$ (14,106)
Denominator:
Weighted-average common units outstanding - basic and diluted		1,594,265		1,538,896
Net loss per unit attributable to common unit holders - basic and diluted		$ (3.33)		$ (9.17)

Net Loss per Share/Unit - Schedule of Potentially Common Shares/Units Excluded from Calculation of Net Loss Per share/Unit (Detail) - shares	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Unvested Restricted Common Unit Awards
Potentially Common Shares/Units Excluded from Calculation of Net Loss Per share/Unit		436,977
Unvested Restricted Common Stock Awards
Potentially Common Shares/Units Excluded from Calculation of Net Loss Per share/Unit	454,833
Stock Options
Potentially Common Shares/Units Excluded from Calculation of Net Loss Per share/Unit	841,510

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended
	Jul. 31, 2017 USD ($) ft²
Asset Purchase Agreement And Related License Agreement [Abstract]
Laboratory and office space to be leased \| ft²	41,346
Operating Lease Annual Rent	$ 3,100,000
Term of lease	10 years
Tenant improvement investment	$ 1,600,000
Letter of credit	$ 1,000,000
Agreement termination term	30 days
Penalty on contract termination	$ 0
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

Related-Party Transactions - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Chief Executive Officer
Schedule of Other Related Party Transactions [Line Items]
Proceeds from repayment of loan			$ 200,000
Interest accrued rate			0.60%
Chief Medical Officer
Schedule of Other Related Party Transactions [Line Items]
Payments to related party	$ 0		$ 0
Reimbursement for a portion of the salary		$ 39,000		$ 136,000
Chief Operating Officer
Schedule of Other Related Party Transactions [Line Items]
Payments to related party	$ 0	$ 0	$ 0	$ 13,000

Subsequent Events - Additional Information (Detail) - $ / shares	Oct. 13, 2017	Sep. 30, 2017	Dec. 31, 2016
Subsequent Event [Line Items]
Common stock ,par value		$ 0.001	$ 0.001
Subsequent Event \| Sales Agreement \| Cowen \| ATM
Subsequent Event [Line Items]
Common stock ,par value	$ 0.001