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.