13G Filing: EcoR1 Capital and Tocagen Inc (TOCA)

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, Schedule 13G or Forms
3, 4 or 5 (and any amendments or supplements thereto) required under section 13(d) or 16(a) of the Securities Exchange Act of 1934,
as amended, in connection with purchases by the undersigned of the common stock of any issuer. For that purpose, the undersigned
hereby constitute and appoint EcoR1 Capital, LLC a Delaware 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 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.

Dated:May 10, 2017