SC 13D

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No.     )*

Dimension Therapeutics, Inc.

(Name of Issuer)

Common Stock, $0.0001 par value per share

(Title of Class of Securities)

25433V105

(CUSIP Number)

Craig L. Slutzkin

New Leaf Venture Management III, L.L.C.,

Times Square Tower

7 Times Square, Suite 3502

New York, NY 10036

(646) 871-6420

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

October 22, 2015

(Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  ¨

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See §240.13d-7 for other parties to whom copies are to be sent.

* The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

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.

CUSIP No. 25433V105 13D Page 2 of 17 Pages

1	NAMES OF REPORTING PERSONS. I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY). New Leaf Ventures III, L.P.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a)  ¨        (b)  ¨
3	SEC USE ONLY
4	SOURCE OF FUNDS (SEE INSTRUCTIONS) WC
5	CHECK IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e) ¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware Limited Partnership
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		7	SOLE VOTING POWER 0 shares
		8	SHARED VOTING POWER 2,059,283 shares
		9	SOLE DISPOSITIVE POWER 0 shares
		10	SHARED DISPOSITIVE POWER 2,059,283 shares
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,059,283 shares
12	CHECK IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) ¨
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 8.0%
14	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) PN

CUSIP No. 25433V105 13D Page 3 of 17 Pages

1	NAMES OF REPORTING PERSONS. I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY). New Leaf Growth Fund I, L.P.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a)  ¨        (b)  ¨
3	SEC USE ONLY
4	SOURCE OF FUNDS (SEE INSTRUCTIONS) WC
5	CHECK IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e) ¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware Limited Partnership
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		7	SOLE VOTING POWER 0 shares
		8	SHARED VOTING POWER 2,059,283 shares
		9	SOLE DISPOSITIVE POWER 0 shares
		10	SHARED DISPOSITIVE POWER 2,059,283 shares
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,059,283 shares
12	CHECK IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) ¨
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 8.0%
14	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) PN

CUSIP No. 25433V105 13D Page 4 of 17 Pages

1	NAMES OF REPORTING PERSONS. I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY). New Leaf Venture Associates III, L.P.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a)  ¨        (b)  ¨
3	SEC USE ONLY
4	SOURCE OF FUNDS (SEE INSTRUCTIONS) AF
5	CHECK IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e) ¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware Limited Partnership
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		7	SOLE VOTING POWER 0 shares
		8	SHARED VOTING POWER 2,059,283 shares
		9	SOLE DISPOSITIVE POWER 0 shares
		10	SHARED DISPOSITIVE POWER 2,059,283 shares
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,059,283 shares
12	CHECK IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) ¨
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 8.0%
14	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) PN

CUSIP No. 25433V105 13D Page 5 of 17 Pages

1	NAMES OF REPORTING PERSONS. I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY). New Leaf Growth Associates I, L.P.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a)  ¨        (b)  ¨
3	SEC USE ONLY
4	SOURCE OF FUNDS (SEE INSTRUCTIONS) AF
5	CHECK IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e) ¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware Limited Partnership
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		7	SOLE VOTING POWER 0 shares
		8	SHARED VOTING POWER 2,059,283 shares
		9	SOLE DISPOSITIVE POWER 0 shares
		10	SHARED DISPOSITIVE POWER 2,059,283 shares
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,059,283 shares
12	CHECK IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) ¨
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 8.0%
14	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) PN

CUSIP No. 25433V105 13D Page 6 of 17 Pages

1	NAMES OF REPORTING PERSONS. I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY). New Leaf Venture Management III, L.L.C.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a)  ¨        (b)  ¨
3	SEC USE ONLY
4	SOURCE OF FUNDS (SEE INSTRUCTIONS) AF
5	CHECK IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e) ¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware Limited Liability Company
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		7	SOLE VOTING POWER 0 shares
		8	SHARED VOTING POWER 2,059,283 shares
		9	SOLE DISPOSITIVE POWER 0 shares
		10	SHARED DISPOSITIVE POWER 2,059,283 shares
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,059,283 shares
12	CHECK IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) ¨
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 8.0%
14	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) PN

CUSIP No. 25433V105 13D Page 7 of 17 Pages

1	NAMES OF REPORTING PERSONS. I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY). Philippe O. Chambon
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a)  ¨        (b)  ¨
3	SEC USE ONLY
4	SOURCE OF FUNDS (SEE INSTRUCTIONS) AF
5	CHECK IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e) ¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION United States citizen
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		7	SOLE VOTING POWER 0 shares
		8	SHARED VOTING POWER 2,059,283 shares
		9	SOLE DISPOSITIVE POWER 0 shares
		10	SHARED DISPOSITIVE POWER 2,059,283 shares
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,059,283 shares
12	CHECK IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) ¨
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 8.0%
14	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) IN

CUSIP No. 25433V105 13D Page 8 of 17 Pages

1	NAMES OF REPORTING PERSONS. I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY). Vijay Lathi
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a)  ¨        (b)  ¨
3	SEC USE ONLY
4	SOURCE OF FUNDS (SEE INSTRUCTIONS) AF
5	CHECK IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e) ¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION United States citizen
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		7	SOLE VOTING POWER 0 shares
		8	SHARED VOTING POWER 2,059,283 shares
		9	SOLE DISPOSITIVE POWER 0 shares
		10	SHARED DISPOSITIVE POWER 2,059,283 shares
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,059,283 shares
12	CHECK IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) ¨
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 8.0%
14	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) IN

CUSIP No. 25433V105 13D Page 9 of 17 Pages

1	NAMES OF REPORTING PERSONS. I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY). Ronald Hunt
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a)  ¨        (b)  ¨
3	SEC USE ONLY
4	SOURCE OF FUNDS (SEE INSTRUCTIONS) AF
5	CHECK IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e) ¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION United States citizen
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		7	SOLE VOTING POWER 0 shares
		8	SHARED VOTING POWER 2,059,283 shares
		9	SOLE DISPOSITIVE POWER 0 shares
		10	SHARED DISPOSITIVE POWER 2,059,283 shares
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,059,283 shares
12	CHECK IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) ¨
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 8.0%
14	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) IN

CUSIP No. 25433V105 13D Page 10 of 17 Pages

1	NAMES OF REPORTING PERSONS. I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY). Jeani Delagardelle
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a)  ¨        (b)  ¨
3	SEC USE ONLY
4	SOURCE OF FUNDS (SEE INSTRUCTIONS) AF
5	CHECK IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e) ¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION United States citizen
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		7	SOLE VOTING POWER 0 shares
		8	SHARED VOTING POWER 2,059,283 shares
		9	SOLE DISPOSITIVE POWER 0 shares
		10	SHARED DISPOSITIVE POWER 2,059,283 shares
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,059,283 shares
12	CHECK IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) ¨
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 8.0%
14	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) IN

CUSIP No. 25433V105 13D Page 11 of 17 Pages

1	NAMES OF REPORTING PERSONS. I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY). Liam Ratcliffe
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a)  ¨        (b)  ¨
3	SEC USE ONLY
4	SOURCE OF FUNDS (SEE INSTRUCTIONS) AF
5	CHECK IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e) ¨
6	CITIZENSHIP OR PLACE OF ORGANIZATION United States citizen
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		7	SOLE VOTING POWER 0 shares
		8	SHARED VOTING POWER 2,059,283 shares
		9	SOLE DISPOSITIVE POWER 0 shares
		10	SHARED DISPOSITIVE POWER 2,059,283 shares
11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,059,283 shares
12	CHECK IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) ¨
13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11) 8.0%
14	TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) IN

CUSIP No. 25433V105 13D Page 12 of 17 Pages

Schedule 13D

Item 1. Security and Issuer.

This statement relates to the Common Stock, $0.0001 par value (the “Common Stock”) of Dimension Therapeutics, Inc. (the “Issuer”) having its principal executive office at 840 Memorial Drive, Cambridge, MA 02139.

Item 2. Identity and Background.

This statement is being filed by:

(a) New Leaf Ventures III, L.P. (“NLV III”) and New Leaf Growth Fund I, L.P. (“NLG I” and, together with NLV III, the “Funds”);

(b) New Leaf Venture Associates III, L.P. (“NLV Associates”), which is the sole general partner of NLV III; New Leaf Growth Associates I, L.P. (“NLG Associates” and, together with NLV Associates, the “GPLPs”), which is the sole general partner of NLG I; and New Leaf Venture Management III, L.L.C. (“NLV Management” and, together with the GPLPs, the “Control Entities”), which is the sole general partner of each of the GPLPs;

(b) Philippe O. Chambon (“Chambon”), Vijay Lathi (“Lathi”), Ronald Hunt (“Hunt”), Jeani Delagardelle (“Delagardelle”) and Liam Ratcliffe (“Ratcliffe” and together with Chambon, Lathi, Hunt and Delagardelle, the “Managing Directors”).

The persons named in this Item 2 are referred to individually herein as a “Reporting Person” and collectively as the “Reporting Persons.”

The address of the principal business office of the Funds, each Control Entity, Chambon, Hunt and Ratcliffe is New Leaf Venture Partners, Times Square Tower, 7 Times Square, Suite 3502, New York, NY 10036. The address of the principal business office of Lathi and Delagardelle is New Leaf Venture Partners, 1200 Park Place, Suite 300, San Mateo, CA 94043.

The principal business of the Funds is to make, hold and dispose of equity and equity-related investments, principally in healthcare, medical device and life sciences companies. The principal business of NLV Associates is to act as the sole general partner of NLV III. The principal business of NLG Associates is to act as the sole general partner of NLG I. The principal business of NLV Management is to act as the sole general partner of the GPLPs. The principal business of each of the Managing Directors is to manage the Control Entities and a number of affiliated partnerships with similar businesses.

During the five years prior to the date hereof, none of the Reporting Persons has been convicted in a criminal proceeding or has been a party to a civil proceeding ending in a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

Each Fund and each GPLP is a limited partnership organized under the laws of the State of Delaware. NLV Management is a limited liability company organized under the laws of the State of Delaware. Each Managing Director is a citizen of the United States.

Item 3. Source and Amount of Funds or Other Consideration.

On October 21, 2015, the Registration Statement on Form S-1 filed with the Securities and Exchange Commission by the Issuer (File No. 333-206911) in connection with its initial public offering of 5,500,000 shares of Common Stock of the Issuer (the “IPO”) was declared effective.

CUSIP No. 25433V105 13D Page 13 of 17 Pages

The closing of the IPO took place on October 22, 2015, and at such closing NLV III purchased an aggregate of 480,769 shares of Common Stock (the “NLV III Securities”) at the IPO price of $13.000 per share. In addition, prior to the IPO, NLV III purchased 2,406,533 shares of Series B Preferred Stock (the “NLV III Series B Stock”) for an aggregate purchase price of $7,749,998.88. Immediately prior to the IPO, the NLV III Series B Stock converted into 823,309 shares of Common Stock of the Issuer. NLV III now holds a total of 1,304,078 shares of the Issuer’s Common Stock (the “NLV III Shares”).

NLG I purchased an aggregate of 480,769 shares of Common Stock (the “NLG I Securities” and, together with the NLV III Securities, the “Securities”), at the IPO price of $13.000 per share. In addition, prior to the IPO, NLG I purchased 802,178 shares of Series B Preferred Stock (the “NLG I Series B Stock”) for an aggregate purchase price of $2,583,334.03. Immediately prior to the IPO, the NLG I Series B stock converted into 274,436 shares of Common Stock of the Issuer. NLG I now holds a total of 755,205 shares of the Issuer’s Common Stock (the “NLG I Shares”).

Collectively, the Funds now hold a total of 2,059,283 shares of the Issuer’s Common stock (the “Firm Shares”).

The working capital of NLV III was the source of the funds for the purchase of the NLV III Securities. The working capital of NLG I was the source of the funds for the purchase of the NLG I Securities. No part of the purchase price of the Securities was represented by funds or other consideration borrowed or otherwise obtained for the purpose of acquiring, holding, trading or voting the Securities.

Item 4. Purpose of Transaction.

The Funds acquired the Common Stock for investment purposes. Depending on market conditions, its continuing evaluation of the business and prospects of the Issuer and other factors, the Funds and other Reporting Persons may dispose of or acquire additional shares of the Issuer. Except as set forth above, none of the Reporting Persons has any present plans which relate to or would result in:

(a) The acquisition by any person of additional securities of the Issuer, or the disposition of securities of the Issuer;

(b) An extraordinary corporate transaction, such as a merger, reorganization or liquidation, involving the Issuer or any of its subsidiaries;

(c) A sale or transfer of a material amount of assets of the Issuer or any of its subsidiaries;

(d) Any change in the present board of directors or management of the Issuer, including any plans or proposals to change the number or term of directors or to fill any existing vacancies on the board;

(e) Any material change in the present capitalization or dividend policy of the Issuer;

(f) Any other material change in the Issuer’s business or corporate structure;

(g) Changes in the Issuer’s charter, bylaws or instruments corresponding thereto or other actions which may impede the acquisition of control of the Issuer by any person;

(h) Causing a class of securities of the Issuer to be delisted from a national securities exchange or to cease to be authorized to be quoted in an inter-dealer quotation system of a registered national securities association;

(i) A class of equity securities of the Issuer becoming eligible for termination of registration pursuant to Section 12(g)(4) of the Securities Exchange Act of 1934; or

(j) Any action similar to any of those enumerated above.

CUSIP No. 25433V105 13D Page 14 of 17 Pages

Item 5. Interest in Securities of the Issuer.

(a) NLV III is the record owner of the NLV III Shares. As the sole general partner of NLV III, NLV Associates may be deemed to own beneficially the NLV III Shares. NLG I is the record owner of the NLG I Shares. As the sole general partner of NLG I, NLG Associates may be deemed to own beneficially the NLG I Shares. As the sole general partner of NLV Associates and NLG Associates, NLV Management may be deemed to own beneficially the Firm Shares.

By virtue of their relationship as affiliated entities, whose Control Entities have overlapping individual controlling persons, each of the Funds may be deemed to share the power to direct the disposition and vote of the Firm Shares. As the individual managers of NLV Management, each of the Managing Directors also may be deemed to own beneficially the Fund Shares.

Each of the Reporting Persons, except for NLV III and NLG I, disclaims beneficial ownership of the NLV III Shares and NLG I Shares, as applicable, except to the extent of their pecuniary interest therein, if any.

The Percentage of outstanding Common Stock of the Issuer which may be deemed to be beneficially owned by such Reporting Person is set forth on Line 13 of such Reporting Person’s cover sheet. Such percentage was calculated based on the (i) 24,879,574 shares of Common Stock reported to be outstanding on the Issuer’s prospectus filed under Rule 424(b)(4), filed with the Securities and Exchange Commission (the “SEC”) on October 21, 2015 and (ii) 825,000 shares of Common Stock pursuant to the option to the underwriters to purchase up to an aggregate of 825,000 shares of Common Stock within 30 days of October 21, 2015.

(b) Regarding the number of shares as to which such person has:

(i) sole power to vote or to direct the vote: See line 7 of cover sheets.

(ii) shared power to vote or to direct the vote: See line 8 of cover sheets.

(iii) sole power to dispose or to direct the disposition: See line 9 of cover sheets.

(iv) shared power to dispose or to direct the disposition: See line 10 of cover sheets.

(c) Except as set forth in Item 3 above, none of the Reporting Persons has effected any transaction in the Common Stock during the last 60 days.

(d) No other person is known to have the right to receive or the power to direct the receipt of dividends from, or any proceeds from the sale of, shares beneficially owned by any of the Reporting Persons.

(e) Not Applicable.

Item 6. Contracts, Arrangements, Undertakings or Relationships with Respect to Securities of the Issuer.

The Funds have entered into a lock-up agreement with the representatives of the underwriters of the IPO pursuant to which the Funds have generally agreed, subject to certain exceptions, not to sell any of the Common Stock of the Issuer or any securities convertible into or exercisable or exchangeable for the Common Stock of the Issuer for a period of 180 days from October 21, 2015 without the prior written consent of the representatives of the underwriters.

CUSIP No. 25433V105 13D Page 15 of 17 Pages

Item 7. Material to be Filed as Exhibits.

Exhibit 99.1 – Agreement regarding filing of joint Schedule 13D.

Exhibit 99.2 – Powers of Attorney regarding Schedule 13D filings.

CUSIP No. 25433V105 13D Page 16 of 17 Pages

SIGNATURE

After reasonable inquiry and to the best of its knowledge and belief, each of the undersigned certifies that the information set forth in this statement is true, complete and correct.

Date: November 2, 2015

NEW LEAF VENTURES III, L.P.
By:	NEW LEAF VENTURE ASSOCIATES III, L.P. General Partner
	By:	NEW LEAF VENTURE MANAGEMENT III, L.L.C. General Partner
		By:	*
			Craig L. Slutzkin
			Chief Financial Officer
NEW LEAF GROWTH FUND I, L.P.
By:	NEW LEAF GROWTH ASSOCIATES I, L.P. General Partner
	By:	NEW LEAF VENTURE MANAGEMENT III, L.L.C. General Partner
		By:	*
			Craig L. Slutzkin
			Chief Financial Officer
NEW LEAF VENTURE ASSOCIATES III, L.P.
By:	NEW LEAF VENTURE MANAGEMENT III, L.L.C. General Partner
	By:	*
		Craig L. Slutzkin
		Chief Financial Officer
NEW LEAF GROWTH ASSOCIATES I, L.P.
By:	NEW LEAF VENTURE MANAGEMENT III, L.L.C. General Partner
	By:	*
		Craig L. Slutzkin
		Chief Financial Officer

CUSIP No. 25433V105 13D Page 17 of 17 Pages

NEW LEAF VENTURE MANAGEMENT III, L.L.C.
	By:	*
		Craig L. Slutzkin
		Chief Financial Officer

    *

Philippe O. Chambon

    *

Vijay Lathi

    *

Ronald Hunt

    *

Jeani Delagardelle

    *

Liam Ratcliffe

/s/ Craig L. Slutzkin

Craig L. Slutzkin

As attorney-in-fact

* This Schedule 13D was executed by Craig L. Slutzkin on behalf of the individuals listed above pursuant to Powers of Attorney, copies of which are attached as Exhibit 99.4.

EX-99.1

CUSIP No. 25433V105 13D Page 1 of 2 Pages

EXHIBIT 99.1

AGREEMENT

Pursuant to Rule 13d-1(k)(1) under the Securities Exchange Act of 1934, the undersigned hereby agree that only one statement containing the information required by Schedule 13D need be filed with respect to the ownership by each of the undersigned of shares of stock of Dimension Therapeutics, Inc.

EXECUTED this 2nd day of November, 2015.

NEW LEAF VENTURES III, L.P.
By:	NEW LEAF VENTURE ASSOCIATES III, L.P. General Partner
	By:	NEW LEAF VENTURE MANAGEMENT III, L.L.C. General Partner
		By:	*
			Craig L. Slutzkin
			Chief Financial Officer
NEW LEAF GROWTH FUND I, L.P.
By:	NEW LEAF GROWTH ASSOCIATES I, L.P. General Partner
	By:	NEW LEAF VENTURE MANAGEMENT III, L.L.C. General Partner
		By:	*
			Craig L. Slutzkin
			Chief Financial Officer
NEW LEAF VENTURE ASSOCIATES III, L.P.
By:	NEW LEAF VENTURE MANAGEMENT III, L.L.C. General Partner
	By:	*
		Craig L. Slutzkin
		Chief Financial Officer
NEW LEAF GROWTH ASSOCIATES I, L.P.
By:	NEW LEAF VENTURE MANAGEMENT III, L.L.C. General Partner
	By:	*
		Craig L. Slutzkin
		Chief Financial Officer

CUSIP No. 25433V105 13D Page 2 of 2 Pages

NEW LEAF VENTURE MANAGEMENT III, L.L.C.
	By:	*
		Craig L. Slutzkin
		Chief Financial Officer

    *

Philippe O. Chambon

    *

Vijay Lathi

    *

Ronald Hunt

    *

Jeani Delagardelle

    *

Liam Ratcliffe

/s/ Craig L. Slutzkin

Craig L. Slutzkin

As attorney-in-fact

* This Schedule 13D was executed by Craig L. Slutzkin on behalf of the individuals listed above pursuant to Powers of Attorney, copies of which are attached as Exhibit 99.2.

EX-99.2

CUSIP No. 25433V105 13D Page 1 of 3 Pages

EXHIBIT 99.2

POWER OF ATTORNEY

KNOWN ALL MEN BY THESE PRESENTS, that the undersigned hereby constitutes and appoints CRAIG L. SLUTZKIN as his true and lawful attorney-in-fact and agent for him and in his name, place and stead, in any and all capacities, to sign any and all documents relating to any and all Securities and Exchange Commission filings which may be required, granting unto said attorney-in-fact and agent full power and authority to do and perform each and every act and thing requisite and necessary to be done in and about the premises, as fully to all intents and purposes as he might or could do in person, thereby ratifying and confirming all that said attorney-in-fact may lawfully do or cause to be done by virtue hereof.

This power shall continue in effect until terminated in writing.

By:	/s/ Vijay K. Lathi
	Vijay K. Lathi

Dated: October 5, 2005

CUSIP No. 25433V105 13D Page 2 of 3 Pages

POWER OF ATTORNEY

KNOWN ALL MEN BY THESE PRESENTS, that the undersigned hereby constitutes and appoints CRAIG L. SLUTZKIN as his/her true and lawful attorney-in-fact and agent for him/her and in his/her name, place and stead, in any and all capacities, to sign any and all documents relating to any and all Securities and Exchange Commission filings which may be required, granting unto said attorney-in-fact and agent full power and authority to do and perform each and every act and thing requisite and necessary to be done in and about the premises, as fully to all intents and purposes as he/she might or could do in person, thereby ratifying and confirming all that said attorney-in-fact may lawfully do or cause to be done by virtue hereof.

This power shall continue in effect until terminated in writing.

By:	/s/ Philippe O. Chambon
	Philippe O. Chambon
By:	/s/ Jeani Delagardelle
	Jeani Delagardelle
By:	/s/ Ronald Hunt
	Ronald Hunt

Dated: September 29, 2006

CUSIP No. 25433V105 13D Page 3 of 3 Pages

POWER OF ATTORNEY

KNOWN ALL MEN BY THESE PRESENTS, that the undersigned hereby constitutes and appoints CRAIG L. SLUTZKIN as his true and lawful attorney-in-fact and agent for him and in his name, place and stead, in any and all capacities, to sign any and all documents relating to any and all Securities and Exchange Commission filings which may be required, granting unto said attorney-in-fact and agent full power and authority to do and perform each and every act and thing requisite and necessary to be done in and about the premises, as fully to all intents and purposes as he might or could do in person, thereby ratifying and confirming all that said attorney-in-fact may lawfully do or cause to be done by virtue hereof.

This power shall continue in effect until terminated in writing.

By:	/s/_ Liam Ratcliffe
	Liam Ratcliffe

Dated: April 9, 2012