X0306 4 2020-07-24 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) 2.23 2020-07-24 4 A 0 1541 0.00 A 2030-07-24 Common Stock 1541 1541 D The options were issued to the Reporting Person pursuant to the Issuer's 2015 Equity Incentive Award Plan in lieu of the non-employee director nominating and governance committee chair cash retainer of $8,000. Such options vest in four equal quarterly installments provided the Reporting Person continues to provide services to the Issuer. The reporting person is a member of Atlas Venture Associates IX, LLC ("AVA 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, L.P. 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/ Ommer Chohan, Attorney-in-fact 2020-07-28