X0306 4 2015-10-27 0 0001592288 Dimension Therapeutics, Inc. DMTX 0001055951 ORBIMED ADVISORS LLC 601 LEXINGTON AVENUE, 54TH FLOOR NEW YORK NY 10022 1 0 1 0 0001615374 OrbiMed Capital GP V LLC 601 LEXINGTON AVENUE, 54TH FLOOR NEW YORK NY 10022 1 0 1 0 0001055949 ISALY SAMUEL D 601 LEXINGTON AVENUE, 54TH FLOOR NEW YORK NY 10022 1 0 1 0 Common Stock 2015-10-27 4 C 0 3250085 A 3250085 I See Footnotes Common Stock 2015-10-27 4 C 0 1947613 A 5197698 I See Footnotes Common Stock 2015-10-27 4 P 0 200000 13 A 5397698 I See Footnotes Series A Preferred Stock 2015-10-27 4 C 0 9500000 D Common Stock 3250085 0 I See Footnotes Series B Preferred Stock 2015-10-27 4 C 0 5692874 D Common Stock 1947613 0 I See Footnotes The Series A Preferred Stock automatically converted into Common Stock, for no additional consideration, on a 2.923-for-1 basis at the closing of the Issuer's initial public offering, and had no expiration date. The Series B Preferred Stock automatically converted into Common Stock, for no additional consideration, on a 2.923-for-1 basis at the closing of the Issuer's initial public offering, and had no expiration date. These securities 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") is the managing member of GP V. Samuel D. Isaly ("Isaly"), a natural person, is the managing member of and owner 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 over the securities held by OPI V and as a result may be deemed to have beneficial ownership over such securities. This report on Form 4 is jointly filed by GP V, Advisors, and Isaly. Each of the Reporting Persons 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. The Reporting Persons have designated a representative, currently Rishi Gupta, an employee of Advisors, to serve on the Issuer's board of directors. This report shall not be deemed an admission that any of the Reporting Persons 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 2015-10-29