X0206 3 2017-08-28 0 0001527599 SYNLOGIC, INC. SYBX 0001055951 ORBIMED ADVISORS LLC 601 LEXINGTON AVE., 54TH FLOOR NEW YORK NY 10022 1 0 0 0 0001055949 ISALY SAMUEL D 601 LEXINGTON AVE., 54TH FLOOR NEW YORK NY 10022 1 0 0 0 0001682115 OrbiMed Capital GP VI LLC 601 LEXINGTON AVE., 54TH FLOOR NEW YORK NY 10022 1 0 0 0 Common Stock 1314996 I See Footnotes These shares are held of record by OrbiMed Private Investments VI, LP ("OPI VI"). OrbiMed Capital GP VI LLC ("GP VI") is the sole general partner of OPI VI, and OrbiMed Advisors LLC ("Advisors"), a registered adviser under the Investment Advisers Act of 1940, as amended, is the sole managing member of GP VI. Samuel D. Isaly ("Isaly"), a natural person, is the managing member of, and holder of a controlling interest in, Advisors. By virtue of such relationships, GP VI, Advisors and Isaly may be deemed to have voting and investment power with respect to the securities held by OPI VI noted above and as a result may be deemed to have beneficial ownership over such securities. The Reporting Persons have designated a representative, currently Chau Khuong, a Private Equity Partner of Advisors, to serve on the Issuer's board of directors. This report on Form 3 is jointly filed by OPI VI, GP VI, Advisors and Isaly. Each of GP VI, Advisors, Isaly and the Reporting Person disclaims beneficial ownership of the securities reported herein for purposes of Rule 16a-1(a) under the Securities Exchange Act of 1934, as amended (the "Exchange Act"), except to the extent of its or his pecuniary interest therein, if any. This report shall not be deemed an admission that any such entity or person is a beneficial owner of such securities for the purpose of Section 16 of the Exchange Act, or for any other purpose. /s/ Samuel D. Isaly 2017-08-30