X0306 4 2019-10-23 0 0001527599 SYNLOGIC, INC. SYBX 0001171002 BELOFF GREGG C/O SYNLOGIC, INC. 301 BINNEY STREET, SUITE 402 CAMBRIDGE MA 02142 0 1 0 0 See Remarks Stock Option (right to buy) 2.30 2019-10-23 4 A 0 50000 0.00 A 2029-10-23 Common Stock 50000 50000 D The option vest with respect to 25% of the shares on a date that is 6 months from the date of grant, with the remaining shares to vest in equal monthly installments over the following 18 months, subject to the Reporting Person continuing to provide services to the Issuer as Interim Chief Financial Officer through each such vesting date. Interim Chief Financial Officer /s/ Daniel Bagliebter, Attorney-in-fact 2019-10-24