X0306 4 2018-06-13 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) 9.36 2018-06-13 4 A 0 10000 0.00 A 2028-06-13 Common Stock 10000 10000 D The option vests and becomes exercisable in full on the earlier of (A) the first anniversary of the date of grant or (B) immediately prior to the next annual meeting of the Corporation's stockholders after the date of grant, subject in each case to the reporting person's continued service as a director of the Corporation. 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/ William J. Bussiere, Jr., Attorney-in-fact 2018-06-14