Kirk Randal J SC 13D/A Synthetic Biologics / KIRK RANDAL J ownership change

Exhibit 5

Joint Filing Agreement

In accordance with Rule 13d-1(k) promulgated under the Securities Exchange Act of 1934, as amended, the persons named below agree to the joint filing on behalf of each of them a Statement on Schedule 13D (including amendments thereto) with regard to the common stock of Synthetic Biologics, Inc. and further agree that this Joint Filing Agreement be included as an exhibit to such joint filings. In evidence thereof, the undersigned, being duly authorized, hereby execute this agreement as of the 19th day of October, 2012.

/s/ Randal J. Kirk
Randal J. Kirk
INTREXON CORPORATION
By:	/s/ Randal J. Kirk
	Randal J. Kirk
	Chief Executive Officer