X0306 4 2018-06-05 0 0001527599 SYNLOGIC, INC. SYBX 0001714563 Brennan Aoife C/O SYNLOGIC, INC. 301 BINNEY STREET, SUITE 402 CAMBRIDGE MA 02142 0 1 0 0 Interim President and CEO Stock Option (right to buy) 9.55 2018-06-05 4 A 0 54000 0.00 A 2028-06-05 Common Stock 54000 54000 D The options vest and become exercisable in substantially equal monthly installments beginning on July 5, 2018 and continuing until June 5, 2022, subject to the Reporting Person continuing to provide services to the Issuer as Interim President and Chief Executive Officer through each such vesting date. /s/ William J. Bussiere, Jr., Attorney-in-fact 2018-06-06