EX-8.1

Exhibit 8.1

April 27, 2020

Tocagen Inc.

4445 Eastgate Mall, Suite 200

San Diego, CA 92121

Ladies and Gentlemen:

We have acted as counsel to Tocagen Inc., a Delaware corporation (the “Acquiror”), in connection with that certain Agreement and Plan of Merger and Reorganization, made and entered into as of February 19, 2020 (the “Merger Agreement”), by and among the Acquiror, Telluride Merger Sub, Inc., a Delaware corporation and direct wholly-owned subsidiary of Acquiror (“Merger Sub”), and Forte Biosciences, Inc., a Delaware corporation (the “Company”), pursuant to which Merger Sub shall be merged with and into the Company (the “Merger”). This opinion is being delivered in connection with the Registration Statement (File No. 333-237371) of the Acquiror on Form S-4 filed on March 25, 2020 with the Securities and Exchange Commission, as amended and supplemented through the date hereof (the “Registration Statement”). This opinion is being rendered pursuant to the requirements of Item 21(a) of Form S-4 under the Securities Act of 1933, as amended. Capitalized terms used herein but not defined shall have the meanings set forth in the Merger Agreement.

In preparing this opinion, we have examined and relied upon the Registration Statement, including the prospectus/offer to exchange, the Merger Agreement, and such other documents as we have deemed necessary or appropriate in order to enable us to render this opinion. In our examination of documents, we have assumed the authenticity of original documents, the accuracy of copies, the genuineness of signatures, and the legal capacity of signatories. We have also assumed that the Merger will be consummated in accordance with the Merger Agreement and as described in the Registration Statement.

In rendering this opinion, we have assumed without investigation or verification that the facts and statements set forth in the Registration Statement and the Merger Agreement are true, correct and complete in all material respects; that the Merger will be completed in accordance with the Merger Agreement and the Registration Statement; that the representations and covenants contained in officer’s certificates delivered to us by the Company, Acquiror and Merger Sub are true and accurate; that there is no change in applicable law between the date hereof and the Effective Time of the Merger; that the Merger will qualify as a statutory merger under the DGCL; that any representation in any of the documents referred to herein that is made “to the best of the knowledge and belief” (or similar qualification) of any person or party is true, correct and complete without such qualification; and that, as to all matters for which a person or entity has represented that such person or entity is not a party to, does not have, or is not aware of, any plan, intention, understanding or agreement, there is no such plan, intention, understanding or agreement. Any inaccuracy in, or breach of, any of the aforementioned statements, representations or assumptions could adversely affect our opinion.

Our opinion is based on existing provisions of the Code, Treasury Regulations, judicial decisions, and rulings and other pronouncements of the Internal Revenue Service as in effect on the date of this opinion, all of which are subject to change (possibly with retroactive effect) or reinterpretation. No assurances can be given that a change in the law on which our opinion is based or the interpretation thereof will not occur or that such change will not affect the opinion expressed herein. We undertake no responsibility to advise of any such developments in the law. In addition, our opinion is being delivered prior to the consummation of the Merger and therefore is prospective and dependent on future events.