FORM 4
[ X ] 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 OF SECURITIES
                                                                                  
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Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940
                      

1. Name and Address of Reporting Person *

Thistle Mary 		2. Issuer Name and Ticker or Trading Symbol

Dimension Therapeutics, Inc. [ DMTX ] 		5. Relationship of Reporting Person(s) to Issuer (Check all applicable)

_____ Director                      _____ 10% Owner
__ X __ Officer (give title below)      _____ Other (specify below)
Chief Operating Officer
(Last)          (First)          (Middle)

C/O DIMENSION THERAPEUTICS, INC., 840 MEMORIAL DRIVE, 4TH FLOOR 		3. Date of Earliest Transaction (MM/DD/YYYY)

11/7/2017
(Street)

CAMBRIDGE, MA 02139
(City)        (State)        (Zip)
4. If Amendment, Date Original Filed (MM/DD/YYYY)

 
6. Individual or Joint/Group Filing (Check Applicable Line)

_ X _ Form filed by One Reporting Person
___ Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1.Title of Security
(Instr. 3) 		2. Trans. Date 2A. Deemed Execution Date, if any 3. Trans. 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 Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities)
1. Title of Derivate Security
(Instr. 3) 		2. Conversion or Exercise Price of Derivative Security 3. Trans. Date 3A. Deemed Execution Date, if any 4. Trans. 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 7. Title and Amount of Securities Underlying Derivative Security
(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)   $3.57   11/7/2017     D         27711      (1) 4/28/2025   Common Stock   27711     (1) 0   D    
Stock Option (Right to Buy)   $3.57   11/7/2017     D         180815      (2) 4/28/2025   Common Stock   180815     (2) 0   D    
Stock Option (Right to Buy)   $4.09   11/7/2017     D         17104      (3) 6/2/2025   Common Stock   17104     (3) 0   D    
Stock Option (Right to Buy)   $7.14   11/7/2017     D         65000      (4) 1/31/2026   Common Stock   65000     (4) 0   D    
Stock Option (Right to Buy)   $1.90   11/7/2017     D         60000      (5) 1/31/2027   Common Stock   60000     (5) 0   D    

Explanation of Responses:
(1)  This stock option, which vests over four years beginning February 2, 2015, at a rate of 25% after 12 months, and in 36 equal monthly installments thereafter, was 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)  This stock option, which vests over four years beginning February 2, 2015, at a rate of 25% after 12 months, and in 36 equal monthly installments thereafter, was assumed by Ultragenyx and converted in accordance with the exchange ratio as set forth in the Merger Agreement.
(3)  This stock option, which vested 2,494 shares on February 2, 2016, and vests in 41 equal monthly installments thereafter, beginning February 3, 2016, was assumed by Ultragenyx and converted in accordance with the exchange ratio as set forth in the Merger Agreement.
(4)  This stock option, which vests in 48 equal monthly installments beginning January 27, 2016, was assumed by Ultragenyx and converted in accordance with the exchange ratio as set forth in the Merger Agreement.
(5)  This stock option, which vests in 48 equal monthly installments beginning January 18, 2017, was assumed by Ultragenyx and converted in accordance with the exchange ratio as set forth in the Merger Agreement.

Reporting Owners
Reporting Owner Name / Address
Relationships
Director 10% Owner Officer Other
Thistle Mary
C/O DIMENSION THERAPEUTICS, INC.
840 MEMORIAL DRIVE, 4TH FLOOR
CAMBRIDGE, MA 02139

Chief Operating Officer

Signatures
/s/ Mary Thistle 11/7/2017
** Signature of Reporting Person Date


Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
* If the form is filed by more than one reporting person, see Instruction 4(b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB control number.
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