FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person * Flynn James E   2. Issuer Name and Ticker or Trading Symbol REGENXBIO Inc. [RGNX] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director __X__ 10% Owner _____ Officer (give title below) __X__ Other (specify below) Possible Member of 10% Group (Last) (First) (Middle) 780 THIRD AVENUE, 37TH FLOOR,  3. Date of Earliest Transaction (Month/Day/Year) 09/22/2015 (Street) NEW YORK, NY 10017 4. If Amendment, Date Original Filed(Month/Day/Year) 6. Individual or Joint/Group Filing(Check Applicable Line) ___ Form filed by One Reporting Person _X_ Form filed by More than One Reporting Person (City) (State) (Zip) Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned

1.Title of Security (Instr. 3) 2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 09/22/2015   C   771,963 A (1) 771,963 I Through Deerfield Private Design III Fund, L.P. (3) (4) Common Stock 09/22/2015   C   397,079 A (2) 1,169,042 I Through Deerfield Private Design III Fund, L.P. (3) (4) Common Stock 09/22/2015   P   87,500 A $ 22 1,256,542 I Through Deerfield Private Design III Fund, L.P. (3) (4) Common Stock 09/22/2015   P   38,500 A $ 22 38,500 I Through Deerfield Partners, L.P. (3) (4) Common Stock 09/22/2015   P   49,000 A $ 22 49,000 I Through Deerfield International Master Fund, L.P. (3) (4)

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
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Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)

1. Title of Derivative Security (Instr. 3)

2. Conversion or Exercise Price of Derivative Security

3. Transaction Date (Month/Day/Year)

3A. Deemed Execution Date, if any (Month/Day/Year)

4. Transaction Code (Instr. 8)

5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5)

6. Date Exercisable and Expiration Date (Month/Day/Year)

7. Title and Amount of Underlying Securities (Instr. 3 and 4)

8. Price of Derivative Security (Instr. 5)

9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)

10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4)

11. Nature of Indirect Beneficial Ownership (Instr. 4)

Code

V

(A)

(D)

Date Exercisable

Expiration Date

Title

Amount or Number of Shares

Series C Convertible Preferred Stock

(1)

09/22/2015

C

771,963

(1)

(1)

Common Stock

771,963

(1)

0

I

Through Deerfield Private Design Fund III, L.P. (3) (4)

Series D Convertible Preferred Stock

(2)

09/22/2015

C

397,079

(2)

(2)

Common Stock

397,079

(2)

0

I

Through Deerfield Private Design Fund III, L.P. (3) (4)

Reporting Owners

Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
Flynn James E 780 THIRD AVENUE, 37TH FLOOR NEW YORK, NY 10017		X		Possible Member of 10% Group
Deerfield Mgmt L.P. 780 THIRD AVENUE 37TH FLOOR NEW YORK, NY 10017		X		Possible Member of 10% Group
DEERFIELD MANAGEMENT CO 780 THIRD AVENUE, 37TH FLOOR NEW YORK, NY 10017		X		Possible Member of 10% Group
Deerfield Mgmt III, L.P. 780 THIRD AVENUE, 37TH FLOOR NEW YORK, NY 10017		X		Possible Member of 10% Group
Deerfield Private Design Fund III, L.P. 780 THIRD AVENUE, 37TH FLOOR NEW YORK, NY 10017		X		Possible Member of 10% Group
Deerfield International Master Fund, L.P. 780 THIRD AVENUE, 37TH FLOOR NEW YORK, NY 10017		X		Possible Member of 10% Group
DEERFIELD PARTNERS, LP 780 THIRD AVENUE 37TH FLOOR NEW YORK, NY 10017		X		Possible Member of 10% Group

Signatures

/s/ Jonathan Isler		09/22/2015
**Signature of Reporting Person		Date

Explanation of Responses:

*	If the form is filed by more than one reporting person, see Instruction 4(b)(v).
**	Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1)	Each share of Series C Convertible Preferred Stock automatically converted into one (1) share of Issuer's common stock upon the closing of the Issuer's initial public offering of common stock.
(2)	Each share of Series D Convertible Preferred Stock automatically converted into one (1) share of Issuer's common stock upon the closing of the Issuer's initial public offering of common stock.
(3)	This Form 4 is being filed by the undersigned as well as the entities listed on the Joint Filer Information Statement attached as an exhibit hereto (the "Reporting Persons"). Deerfield Mgmt III, L.P. is the general partner of Deerfield Private Design Fund III, L.P. ("Fund III"). Deerfield Mgmt, L.P. is the general partner of Deerfield Partners, L.P., and Deerfield International Master Fund, L.P. (collectively with Fund III, the "Funds"). Deerfield Management Company, L.P. is the investment manager of the Funds. James E. Flynn is the sole member of the general partner of each of Deerfield Mgmt, L.P., Deerfield Mgmt III, L.P. and Deerfield Management Company, L.P.
(4)	In accordance with Instruction 4 (b)(iv) to Form 4, the entire amount of the Issuer's securities held by the Funds is reported herein. For purposes of Section 16 of the Securities Exchange Act of 1934, as amended, each Reporting Person disclaims beneficial ownership of any such securities, except to the extent of his/its indirect pecuniary interest therein, if any, and this report shall not be deemed an admission that such Reporting Person is the beneficial owner of such securities for purposes of Section 16 or otherwise.
Remarks: Jonathan Isler, Attorney-in-Fact: Power of Attorney, which is hereby incorporated by reference to Exhibit 24 to a Form 3 with regard to Avalanche Biotechnologies, Inc. filed with the Securities and Exchange Commission on July 30, 2014 by Deerfield Mgmt III, L.P., Deerfield Mgmt, L.P., Deerfield Management Company, L.P., Deerfield Special Situations Fund, L.P., Deerfield Special Situations International Master Fund, L.P., Deerfield Private Design Fund III, L.P. and James E. Flynn.

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