POWER OF ATTORNEY

        The undersigned individual (the "Reporting Person") hereby constitutes
and appoints each of Sara Garon Berl and Albert Vanderlaan, signing singly, with
full power of substitution, as the Reporting Person's true and lawful attorney
in fact to:

        (1)     prepare and file on behalf of such Reporting Person any and all
reports, notices, communications and other documents (including, but not limited
to, reports on Schedule 13D, Schedule 13G, Form 13-F, Form 3, Form 4 and Form 5)
that such Reporting Person may be required to file with the SEC pursuant to the
Securities Act of 1933, as amended (together with the implementing regulations
thereto, the "Act") and the Securities Exchange Act of 1934, as amended
(together with the implementing regulations thereto, the "Exchange Act")
(collectively, the "Reports") with respect to the Reporting Person's ownership
of, or transactions in, the securities of REGENXBIO Inc. (the "Company"),
(whether directly or indirectly owned) by such Reporting Person;

        (2)     do and perform any and all acts for and on behalf of the
Reporting Person which may be necessary or desirable to complete and execute any
such Reports, complete and execute any amendment or amendments thereto, and
timely file such form with the SEC and any stock exchange or similar authority;
and

        (3)     take any other action of any type whatsoever in connection with
the foregoing which, in the opinion of such attorney in fact, may be of benefit
to, in the best interest of, or legally required by, the Reporting Person, it
being understood that the documents executed by such attorney in fact on behalf
of the Reporting Person pursuant to this Power of Attorney shall be in such form
and shall contain such terms and conditions as such attorney in fact may approve
in such attorney in fact's discretion.

        The Reporting Person hereby grants to such attorney in fact full power
and authority to do and perform any and every act and thing whatsoever
requisite, necessary, or proper to be done in the exercise of any of the rights
and powers herein granted, as fully to all intents and purposes as the Reporting
Person might or could do if personally present, with full power of substitution
or revocation, hereby ratifying and confirming all that such attorney in fact,
or such attorney in fact's substitute or substitutes, shall lawfully do or cause
to be done by virtue of this power of attorney and the rights and powers herein
granted. The Reporting Person acknowledges that the foregoing attorney in fact,
in serving in such capacity at the request of the Reporting Person, is not
assuming any of the Reporting Person's responsibilities to comply with Section
16 of the Securities Exchange Act of 1934.

        This Power of Attorney with respect to the Reporting Person shall remain
in full force and effect until such Reporting Person is no longer required to
file any Reports with respect to the Reporting Person's ownership of, or
transactions in, the securities of the Company, unless earlier revoked in a
signed writing delivered to the foregoing attorney in fact.

        IN WITNESS WHEREOF, the undersigned have caused this Power of Attorney
to be executed as of this 11th day of August, 2016.

                                        /s/ Daniel Tasse
                                        ---------------------------------------
                                        Daniel Tasse