X0306 4 2013-09-24 1 0001280600 ACCELERON PHARMA INC XLRN 0001055951 ORBIMED ADVISORS LLC 601 LEXINGTON AVENUE, 54TH FLOOR NEW YORK NY 10022 0 0 0 1 Former 10% Owner 0001281668 ORBIMED CAPITAL GP II LLC 601 LEXINGTON AVENUE, 54TH FLOOR NEW YORK\ NY 10022 0 0 0 1 Former 10% Owner 0001055949 ISALY SAMUEL D 601 LEXINGTON AVENUE, 54TH FLOOR NEW YORK NY 10022 0 0 0 1 Former 10% Owner Common Stock 2013-09-24 4 C 0 1127346 A 1133171 I See footnote Common Stock 2013-09-24 4 C 0 422101 A 424282 I See footnote Common Stock 2013-09-24 4 C 0 175419 A 1308590 I See footnote Common Stock 2013-09-24 4 C 0 65680 A 489962 I See footnote Common Stock 2013-09-24 4 C 0 23442 A 1332032 I See footnote Common Stock 2013-09-24 4 C 0 8777 A 498739 I See footnote Common Stock 2013-09-24 4 C 0 133930 A 1465962 I See footnote Common Stock 2013-09-24 4 C 0 50145 A 548884 I See footnote Common Stock 2013-09-24 4 C 0 35873 A 1501835 I See footnote Common Stock 2013-09-24 4 C 0 13431 A 562315 I See footnote Common Stock 2013-09-24 4 C 0 38924 A 1540759 I See footnote Common Stock 2013-09-24 4 C 0 14574 A 576889 I See footnote Series B Convertible Preferred Stock 2013-09-24 4 C 0 1127346 D Common Stock 1127346 0 I See footnote Series B Convertible Preferred Stock 2013-09-24 4 C 0 422101 D Common Stock 422101 0 I See footnote Series C Convertible Preferred 2013-09-24 4 C 0 175419 D Common Stock 175419 0 I See footnote Series C Convertible Preferred Stock 2013-09-24 4 C 0 65680 D Common Stock 65680 0 I See footnote Series D Convertible Preferred 2013-09-24 4 C 0 23442 D Common Stock 23442 0 I See footnote Series D Convertible Preferred Stock 2013-09-24 4 C 0 8777 D Common Stock 8777 0 I See footnote Series E Convertible Preferred 2013-09-24 4 C 0 83097 D Common Stock 133930 0 I See footnote Series E Convertible Preferred Stock 2013-09-24 4 C 0 31113 D Common Stock 50145 0 I See footnote Series E Convertible Preferred 2013-09-24 4 C 0 22612 D Common Stock 35873 0 I See footnote Series E Convertible Preferred 2013-09-24 4 C 0 8466 D Common Stock 13431 0 I See footnote Series F Convertible Preferred 2013-09-24 4 C 0 38924 D Common Stock 38924 0 I See footnote Series F Convertible Preferred 2013-09-24 4 C 0 14574 D Common Stock 14574 0 I See footnote Represents shares of Common Stock acquired upon the automatic conversion of Series B convertible preferred stock upon the closing of the Issuer's initial public offering of Common Stock. The Series B convertible preferred stock did not have an expiration date. The Series B convertible preferred stock automatically converted into shares of Common Stock immediately upon the closing of the Issuer's initial public offering of Common Stock, at a conversion ratio of 1:1, for no additional consideration. Represents shares of Common Stock acquired upon the automatic conversion of Series C convertible preferred stock upon the closing of the Issuer's initial public offering of Common Stock. The Series C convertible preferred stock did not have an expiration date. The Series C convertible preferred stock automatically converted into shares of Common Stock immediately upon the closing of the Issuer's initial public offering of Common Stock, at a conversion ratio of 1:1, for no additional consideration. Represents shares of Common Stock acquired upon the automatic conversion of Series D convertible preferred stock upon the closing of the Issuer's initial public offering of Common Stock. The Series D convertible preferred stock did not have an expiration date. The Series D convertible preferred stock automatically converted into shares of Common Stock immediately upon the closing of the Issuer's initial public offering of Common Stock, at a conversion ratio of 1:1, for no additional consideration. Represents shares of Common Stock acquired upon the automatic conversion of Series E convertible preferred stock upon the closing of the Issuer's initial public offering of Common Stock. The Series E convertible preferred stock did not have an expiration date. The Series E convertible preferred stock automatically converted into shares of Common Stock immediately upon the closing of the Issuer's initial public offering of Common Stock, at a conversion ratio of 1.61 shares of Common Stock for every 1 share of Series E convertible preferred stock, for no additional consideration. Represents shares of Common Stock acquired upon the automatic conversion of Series E convertible preferred stock upon the closing of the Issuer's initial public offering of Common Stock. The Series E convertible preferred stock did not have an expiration date. The Series E convertible preferred stock automatically converted into shares of Common Stock immediately upon the closing of the Issuer's initial public offering of Common Stock, at a conversion ratio of 1.59 shares of Common Stock for every 1 share of Series E convertible preferred stock, for no additional consideration. Represents shares of Common Stock acquired upon the automatic conversion of Series F convertible preferred stock upon the closing of the Issuer's initial public offering of Common Stock. The Series F convertible preferred stock did not have an expiration date. The Series F convertible preferred stock automatically converted into shares of Common Stock immediately upon the closing of the Issuer's initial public offering of Common Stock, at a conversion ratio of 1:1, for no additional consideration. These securities are directly owned by OrbiMed Private Investments II, LP ("OPI II") and may be deemed to be indirectly beneficially owned by OrbiMed Capital GP II LLC ("GP II"), OrbiMed Advisors LLC ("Advisors") and Samuel D. Isaly ("Isaly"). GP II is the sole general partner of OPI II. Advisors, a registered adviser under the Investment Advisers Act of 1940, as amended, is the sole managing member of GP II. By virtue of such relationships, GP II and Advisors may be deemed to have voting and investment power over the securities held by OPI II noted above. Accordingly, GP II and Advisors may be deemed to have beneficial ownership of such securities. Isaly, a natural person, may also be deemed to have voting and investment power over the securities held by OPI II noted above in his capacity as the managing member of, and owner of a controlling interest in, Advisors. Accordingly, Isaly may be deemed to have beneficial ownership of such securities. These securities are directly owned by OrbiMed Private Investments II (QP), LP ("OPI II QP") and may be deemed to be indirectly beneficially owned by GP II, Advisors and Isaly. GP II is the sole general partner of OPI II QP. Advisors is the sole managing member of GP II. Isaly is the managing member of, and owner of a controlling interest in, Advisors. By virtue of such relationships, GP II, Advisors and Isaly may be deemed to have voting and investment power over the securities held by OPI II QP noted above. Accordingly, GP II, Advisors and Isaly may be deemed to have beneficial ownership of such securities. This report on Form 4 is jointly filed by GP II, Advisors and Isaly. GP II, Advisors and Isaly designated a representative, Carl L. Gordon ("Gordon"), a member of Advisors, to serve on the Issuer's board of directors. On September 18, 2013 and in connection with the Issuer's initial public offering, Gordon resigned from the Issuer's board of directors. Each of GP II, Advisors, Isaly and Gordon 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 of them is the beneficial owner of such securities for purposes of Section 16 of the Exchange Act or for any other purpose. /s/ Samuel D. Isaly 2013-09-26