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SYNLOGIC, INC. 301 BINNEY ST, SUITE 402 CAMBRIDGE, MA 02142 V16060-P86989 You invested in SYNLOGIC, INC. and it’s time to vote! You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy material for the stockholder meeting to be held on June 15, 2023. Get informed before you vote View the Notice and Proxy Statement and Form 10-K online OR you can receive a free paper or email copy of the material(s) by requesting prior to June 1, 2023. If you would like to request a copy of the material(s) for this and/or future stockholder meetings, you may (1) visit, (2) call 1-800-579-1639 or (3) send an email to If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy. Smartphone users Point your camera here and vote without entering a control number Virtually at Vote Virtually at the Meeting* June 15, 2023 8:00 AM, EDT *Please check the meeting materials for any special requirements for meeting attendance. V1.2 graphic For complete information and to vote, visit Control # 


Vote at THIS IS NOT A VOTABLE BALLOT This is an overview of the proposals being presented at the upcoming stockholder meeting. Please follow the instructions on the reverse side to vote these important matters. Voting Items Board Recommends 1. Election of Class II Directors Nominees: 01) Peter Barrett, Ph.D. 02) Michael Heffernan 03) Lisa Kelly-Croswell 04) Edward Mathers For 2. To approve by an advisory vote the compensation of the Company’s named executive officers, as disclosed in the proxy statement. For 3. Ratification of appointment of KPMG LLP as the Company’s independent registered public accounting firm for fiscal year ending December 31, 2023. For 4. To approve an amendment to the Company’s Amended and Restated Certificate of Incorporation to reflect new Delaware law provisions regarding officer exculpation. For NOTE: Such other business as may properly come before the meeting or any adjournment thereof. Prefer to receive an email instead? While voting on, be sure to click “Delivery Settings”. V16061-P86989 graphi