PROOF PROOF   Dimension TherapeuTics, inc. INCORPORATED UNDER THE LAWS OF THE STATE OF   DELAWARE COMMON STOCK SEE REVERSE FOR CERTAIN DEFINITIONS this Certifies   that: PROOF is the owner of FULLY PAID AND NON-ASSESSABLE SHARES OF COMMON   STOCK OF $0.0001 PAR VALUE EACH OF Dimension TherapeuTics, inc. transferable   on the books of the Corporation in person or by attorney upon surrender of   this certificate duly endorsed or assigned. This certificate and the shares   represented hereby are subject to the laws of the State of Delaware, and to   the Certificate of Incorporation and Bylaws of the Corporation, as now or   hereafter amended. This certificate is not valid until countersigned by the   Transfer Agent. WITNESS the facsimile seal of the Corporation and the   facsimile signatures of its duly authorized officers. DateD: SECRETARY   PRESIDENT COUNTERSIGNED AND REGISTERED: AMERICAN STOCK TRANSFER & TRUST   COMPANY, LLC BROOKLYN, NY TRANSFER AGENT AND REGISTRAR BY: AUTHORIZED   SIGNATURE CUSIP25433V 10 5 SHARES NUMBER
    




 









	

    	
THE CORPORATION   WILL FURNISH TO ANY STOCKHOLDER, UPON REQUEST AND WITHOUT CHARGE, A FULL   STATEMENT OF THE DESIGNA-TIONS, RELATIVE RIGHTS, PREFERENCES AND LIMITATIONS   OF THE SHARES OF EACH CLASS AND SERIES AUTHORIZED TO BE ISSUED, SO FAR AS THE   SAME HAVE BEEN DETERMINED, AND OF THE AUTHORITY, IF ANY, OF THE BOARD TO   DIVIDE THE SHARES INTO CLASSES OR SERIES AND TO DETERMINE AND CHANGE THE   RELATIVE RIGHTS, PREFERENCES AND LIMITATIONS OF ANY CLASS OR SERIES. SUCH   REQUEST MAY BE MADE TO THE SECRETARY OF THE CORPORATION OR TO THE TRANSFER AGENT   NAMED ON THIS CERTIFICATE. The following abbreviations, when used in the   inscription on the face of this certificate, shall be construed as though   they were written out in full according to applicable laws or regulations:   TEN COM TEN ENT JT TEN - - - as tenants in common as tenants by the   entireties as joint tenants with right of survivorship and not as tenants in   common UNIF GIFT MIN ACT - ....................Custodian....................   (Cust) (Minor) under Uniform Gifts to Minors Act................... (State)   Additional abbreviations may also be used though not in the above list. For   Value Received, hereby sell, assign and transfer unto PLEASE INSERT SOCIAL   SECURITY OR OTHER IDENTIFYING NUMBER OF ASSIGNEE (PLEASE PRINT OR TYPEWRITE   NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE) Shares of the stock   represented by the within Certificate, and do hereby irrevocably constitute   and appoint Attorney to transfer the said stock on the books of the within   named Corporation with full power of substitution in the premises. Dated   NOTICE: THE SIGNATURE(S) TO THIS ASSIGNMENT MUST CORRESPOND WITH THE NAME(S)   AS WRITTEN UPON THE FACE OF THE CERTIFICATE, IN EVERY PARTICULAR, WITHOUT   ALTERATION OR ENLARGEMENT OR ANY CHANGE WHATSOEVER. Signature(s) Guaranteed   By The Signature(s) must be guaranteed by an eligible guarantor institution   (Banks, Stockbrokers, Savings and Loan Associations and Credit Unions with   membership in an approved Signature Guarantee Medallion Program), pursuant to   SEC Rule 17Ad-15. COLUMBIA PRINTING SERVICES, LLC - www.stockinformation.com