Sec Form 4 Filing - Dybbs Michael @ Dimension Therapeutics, Inc. - 2017-11-07

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Dybbs Michael 2. Issuer Name and Ticker or Trading Symbol Dimension Therapeutics, Inc. [ DMTX] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director _____ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O DIMENSION THERAPEUTICS, INC., 840 MEMORIAL DRIVE, 4TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 11/07/2017 (Street) CAMBRIDGE, MA02139 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Stock Option (Right to Buy) $ 7.08 11/07/2017 D 32,069 ( 1 ) 05/18/2026 Common Stock 32,069 ( 1 ) 0 D Stock Option (Right to Buy) $ 1.15 11/07/2017 D 17,000 ( 2 ) 05/24/2027 Common Stock 17,000 ( 2 ) 0 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Dybbs Michael C/O DIMENSION THERAPEUTICS, INC. 840 MEMORIAL DRIVE, 4TH FLOOR CAMBRIDGE, MA02139 X Signatures /s/ Mary Thistle, as Attorney-in-Fact for Michael Dybbs 11/07/2017 Signature of Reporting Person Date

Explanation of Responses:

( 1 )These options were assumed by Ultragenyx Pharmaceutical Inc. ("Ultragenyx"), and converted in accordance with the exchange ratio as set forth in the Agreement and Plan of Merger (the "Merger Agreement"), dated October 2, 2017, between the Issuer, Ultragenyx and Mystic River Merger Sub Inc., a direct, wholly-owned subsidiary of Ultragenyx. ( 2 )These options, which vest in full on the earlier of May 25, 2018 or the Issuer's next annual meeting of stockholders, subject to the director's continued service on the Board, were assumed by Ultragenyx and converted in accordance with the exchange ratio as set forth in the Merger Agreement.

Remarks: The Reporting Person previously reported an indirect interest in shares held directly by New Leaf Ventures III, L.P. ("NLV-III") and New Leaf Growth Fund I, L.P. ("NLGF"), with respect to which the Reporting Person disclaimed Section 16 beneficial ownership except to the extent of his/her pecuniary interest therein, if any. New Leaf Venture Associates III, L.P. ("NLVA-III") is the general partner of NLV-III and New Leaf Venture Management III, L.L.C. ("NLVM-III") is the general partner of NLVA-III. New Leaf Growth Associates I, L.P. ("NLGA") is the general partner of NLGF and NLVM-III is the general partner of NLGA. Reporting Person was employed by New Leaf Venture Partners, L.L.C. ("NLVP"), which is affiliated with the above entities. In 2016, the Reporting Person separated from NLVP and is no longer affiliated with either NLV-III or NLGF and accordingly is no longer reporting any indirect interest in the shares held directly by either NLV-III or NLGF.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

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