X0306 4 2019-01-02 0 0001527599 SYNLOGIC, INC. SYBX 0001291895 Barrett Peter C/O SYNLOGIC, INC. 301 BINNEY STREET, SUITE 402 CAMBRIDGE MA 02142 1 0 0 0 Stock Option (right to buy) 7.62 2019-01-02 4 A 0 17380 4.39 A 2029-01-02 Common Stock 17380 17380 D The options were issued to the Reporting Person pursuant to the Issuer's Amended and Restated Non-Employee Director Compensation Program and 2015 Equity Incentive Award Plan in lieu of the non-employee director board and committee cash retainers of $76,250 in the aggregate. Such options vest in four quarterly installments on the last day of each calendar quarter during 2019 provided the Reporting Person continues to provide services to the Issuer. The reporting person is a member of Atlas Venture Associates IX, LLC and is obligated to transfer the economic benefit, if any, received upon the sale of the shares issuable upon exercise of the equity grants to Atlas Venture Advisors, LLC. As such, the Reporting Person disclaims beneficial ownership of the securities reported herein for purposes of Section 16 of the Securities Exchange Act of 1934, as amended, except to the extent of his pecuniary interest therein, if any. /s/ Frank Castellucci, Attorney-in-fact 2019-04-09