X0306 4 2017-12-29 0 0001527599 SYNLOGIC, INC. SYBX 0001291895 Barrett Peter C/O SYNLOGIC, INC. 200 SIDNEY STREET, SUITE 320 CAMBRIDGE MA 02139 1 0 0 0 Stock Option (right to buy) 9.70 2017-12-29 4 A 0 17143 0.00 A 2027-12-29 Common Stock 17143 17143 D Stock Option (right to buy) 10.80 2018-01-02 4 A 0 11570 6.60 A 2028-01-02 Common Stock 11570 11570 D The options vest and become exercisable in substantially equal installments on each of the first three anniversaries of August 28, 2017 subject to the Reporting Person continuing to provide services to the Issuer through each such vesting date. 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 2018 provided the Reporting Person continues to provide services to the Issuer. /s/ William J. Bussiere, Jr., Attorney-in-fact 2018-01-03