DMTX Dimension Therapeutics, Inc.

On July 21, 2017, Jean Franchi resigned as Chief Financial Officer, Secretary and Treasurer of Dimension Therapeutics, Inc. (the “Company”), effective July 28, 2017.

On July 24, 2017, the Company also entered into a separation agreement (the “Separation Agreement”) with Ms. Franchi in connection with her separation from service. Pursuant to the terms of the Separation Agreement and in consideration of a customary release of any claims by Ms. Franchi against the Company, Ms. Franchi will be entitled to severance payments in the aggregate amount equal to nine months of her annual base salary to be paid in monthly installments beginning on the first payroll period after the Separation Agreement becomes effective, continuation of healthcare benefits for up to nine months, and 100% vesting of all time-based equity awards with an exercise period extended until nine months following the separation from service. Additionally, if a change of control occurs during the nine-month period following Ms. Franchi’s separation from service, Ms. Franchi will be entitled to extended continuation of healthcare benefits until the twelfth month following her separation from service, and the foregoing payments that are not yet paid will be accelerated and paid along with an amount equal to three months of her annual base pay as well as her prorated target bonus.

The foregoing summary of the Separation Agreement is qualified in its entirety by the full text of the Separation Agreement, which is filed herewith as Exhibit 10.1 and incorporated herein by reference.

Effective July 28, 2017, Mary Thistle, the Company’s Chief Operating Officer will serve as the Company’s principal financial and accounting officer. Ms. Thistle, age 57, joined the Company as its chief business officer in February 2015 and became the Company’s chief operating officer in December 2016. From January 2014 until she joined the Company, Ms. Thistle served as senior vice president of business development at Cubist Pharmaceuticals Inc. (“Cubist”). Ms. Thistle joined Cubist in January 2009 and served as its senior director, business development from 2009 to 2012 and then its vice president, business development from 2012 to 2013. From 1997 through 2008, Ms. Thistle served as vice president and then senior vice president at ViaCell Inc., which was acquired by PerkinElmer LAS in 2007. Ms. Thistle holds a B.S. in business and accounting from the University of Massachusetts and has successfully completed the Uniform CPA Examination.

Since the beginning of the Company’s last fiscal year, the Company has not engaged in any transaction in which Ms. Thistle had a direct or indirect material interest within the meaning of Item 404(a) of Regulation S-K under the U.S. Securities Act of 1933.