13G Filing: Palo Alto Investors and Epizyme Inc. (NASDAQ:EPZM)

EXHIBIT A

AGREEMENT REGARDING JOINT FILING

OF STATEMENT ON SCHEDULE 13D OR 13G

The undersigned agree to file jointly with
the Securities and Exchange Commission (the “SEC”) any and all statements on Schedule 13D or Schedule 13G (and any amendments
or supplements thereto) required under section 13(d) of the Securities Exchange Act of 1934, as amended, in connection with purchases
and sales by the undersigned of the securities of any issuer until such time as the undersigned file with the SEC a statement terminating
this Agreement Regarding Joint Filing of Statement on Schedule 13D or 13G. For that purpose, the undersigned hereby constitute
and appoint Palo Alto Investors, LLC, a California limited liability company, as their true and lawful agent and attorney-in-fact,
with full power and authority for and on behalf of the undersigned to prepare or cause to be prepared, sign, file with the SEC
and furnish to any other person all certificates, instruments, agreements and documents necessary to comply with section 13(d)
and section 16(a) of the Securities Exchange Act of 1934, as amended, in connection with said purchases and sales, and to do and
perform every act necessary and proper to be done incident to the exercise of the foregoing power, as fully as the undersigned
might or could do if personally present, until such time as the undersigned file with the SEC a statement terminating this Agreement
Regarding Joint Filing of Statement on Schedule 13D or 13G.

Dated: April 3, 2017