FORM 3

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

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1. Name and Address of Reporting Person* Dybbs Michael (Last) (First) (Middle) C/O NEW LEAF VENTURE PARTNERS 7 TIMES SQUARE, SUITE 3502 (Street) NEW YORK NY 10036 (City) (State) (Zip)	2. Date of Event Requiring Statement (Month/Day/Year) 10/21/2015	3. Issuer Name and Ticker or Trading Symbol Dimension Therapeutics, Inc. [ DMTX ]
		4. Relationship of Reporting Person(s) to Issuer (Check all applicable) X Director 10% Owner Officer (give title below) Other (specify below)	5. If Amendment, Date of Original Filed (Month/Day/Year)
			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 Beneficially Owned
1. Title of Security (Instr. 4)	2. Amount of Securities Beneficially Owned (Instr. 4)	3. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	4. Nature of Indirect Beneficial Ownership (Instr. 5)

Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 4)	2. Date Exercisable and Expiration Date (Month/Day/Year)		3. Title and Amount of Securities Underlying Derivative Security (Instr. 4)		4. Conversion or Exercise Price of Derivative Security	5. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	6. Nature of Indirect Beneficial Ownership (Instr. 5)
	Date Exercisable	Expiration Date	Title	Amount or Number of Shares
Series B Preferred Stock	(1)	(1)	Common Stock	1,097,745(1)	(1)	I	See Footnote(2)

Explanation of Responses:

1. 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.

2. These securities are held by New Leaf Ventures III, L.P. ("NLV-III") and New Leaf Growth Fund I, L.P. ("NLGF"). 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 is employed by New Leaf Venture Partners, L.L.C., which is affiliated with the above entities and disclaims beneficial ownership of the shares held by NLV-III and NLGF except to the extent of his pecuniary interests therein if any.

Remarks:

Exhibit 24.1: Limited Power of Attorney

	/s/ Mary Thistle, as Attorney-in-Fact for Michael Dybbs	10/21/2015
	** 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 5 (b)(v).
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