X0206 3 2015-10-21 0 0001592288 Dimension Therapeutics, Inc. DMTX 0001055951 ORBIMED ADVISORS LLC 601 LEXINGTON AVENUE, 54TH FLOOR NEW YORK NY 10022 1 0 1 0 Series A Preferred Stock Common Stock 3250085 I See Footnotes Series B Preferred Stock Common Stock 1947613 I See Footnotes These securities are shares of Series A Preferred Stock (the "Series A Shares") of the Issuer, and are convertible at any time into shares of the Issuer's Common Stock at the holders' election and automatically upon the closing of the Issuer's initial public offering, into the number of shares of Common Stock shown in column 3 above. These securities are shares of Series B Preferred Stock (the "Series B Shares") of the Issuer, and are convertible at any time into shares of the Issuer's Common Stock at the holders' election and automatically upon the closing of the Issuer's initial public offering, into the number of shares of Common Stock shown in column 3 above. 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 3 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 2015-10-21