Exhibit I

Each of the undersigned hereby agrees to be included in the filing of the Amendment No. 2 to the Schedule 13D dated December 31, 2003 with respect to the issued and outstanding Common Stock of Lexicon Genetics Incorporated beneficially owned by each of the undersigned, respectively.

Dated: December 31, 2003

ESTATE OF GORDON A. CAIN
By:	/s/    MARY H. CAIN
	Mary H. Cain Co-Executor
By:	/s/    JAMES D. WEAVER
	James D. Weaver Co-Executor
	/s/    MARY H. CAIN
	Mary H. Cain
	/s/    JAMES D. WEAVER
	James D. Weaver
	/s/    WILLIAM A. MCMINN
	William A. McMinn