X0206 3 2017-10-04 0 0001649904 RHYTHM PHARMACEUTICALS, INC. RYTM 0001055951 ORBIMED ADVISORS LLC 601 LEXINGTON AVE., 54TH FLOOR NEW YORK NY 10022 0 0 1 0 0001615374 OrbiMed Capital GP V LLC 601 LEXINGTON AVE., 54TH FLOOR NEW YORK NY 10022 0 0 1 0 0001055949 ISALY SAMUEL D 601 LEXINGTON AVE., 54TH FLOOR NEW YORK NY 10022 0 0 1 0 Series A Convertible Preferred Stock 0 Common Stock 2546295 I See Footnote Upon the closing of the Issuer's initial public offering (the "IPO"), each share of the Issuer's series A convertible preferred stock (the "Series A Preferred") will automatically convert into shares of the Issuer's common stock on a 9.17-for-one basis for no consideration. The Series A Preferred has no expiration date. The shares of Series A Preferred reported are held of record by OrbiMed Private Investments V, LP ("OPI V"). OrbiMed Capital GP V LLC ("GP V") is the sole general partner of OPI V and OrbiMed Advisors LLC ("Advisors"), a registered adviser under the Investment Advisers Act of 1940, as amended, is the sole managing member of GP V. 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 V, Advisors and Isaly may be deemed to have voting and investment power with respect to the shares of Series A Preferred held by OPI V and as a result, may be deemed to have beneficial ownership over such shares of Series A Preferred. This report on Form 3 is jointly filed by GP V, Advisors and Isaly. Each of OPI V, GP V, Advisors and Isaly 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 2017-10-04