13G Filing: Foresite Capital Fund III, L.P. and Tocagen Inc (TOCA)

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exhibit A

Agreement of Joint Filing

The undersigned hereby
agree that a single Schedule 13G (or any amendment thereto) relating to the common stock of the Issuer shall be filed on behalf
of each of the undersigned and that this Agreement shall be filed as an exhibit to such Schedule 13G.

Date:  April 24, 2017
FORESITE CAPITAL FUND III, L.P.
By: FORESITE CAPITAL MANAGEMENT III, LLC
Its: General Partner
By: /s/ James Tananbaum
Name: James Tananbaum
Title: Managing Member
FORESITE CAPITAL MANAGEMENT III, LLC
By: /s/ James Tananbaum
Name: James Tananbaum
Title: Managing Member
JAMES TANANBAUM
By: /s/ James Tananbaum
Name: James Tananbaum

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