X0306 4 2017-08-28 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) 12.07 2017-08-28 4 A 0 2857 0.00 A 2027-08-28 Common Stock 2857 2857 D The option vests and becomes exercisable in substantially equal installments on each of the first three anniversaries of the Transaction Date, subject to the Reporting Person continuing to provide services to the Issuer through such vesting date. 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/ Peter Barrett 2017-08-30