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                                                                   EXHIBIT 99.1


                                  CERTIFICATION

         Pursuant to Section 906 of the Sarbanes-Oxley Act of 2002 (18 U.S.C.
1350, as adopted), Arthur T. Sands, M.D., Ph.D., Chief Executive Officer of
Lexicon Genetics Incorporated ("Lexicon"), and Julia P. Gregory, Chief Financial
Officer of Lexicon, each hereby certify that:

         1.  Lexicon's Quarterly Report on Form 10-Q for the period ended March
             31, 2003, and to which this Certification is attached as Exhibit
             99.1 (the "Periodic Report"), fully complies with the requirements
             of section 13(a) or section 15(d) of the Securities Exchange Act of
             1934, and

         2.  The information contained in the Periodic Report fairly presents,
             in all material respects, the financial condition and results of
             operations of Lexicon.

         IN WITNESS WHEREOF, the undersigned have set their hands hereto as of
the 9th day of May, 2003.

Date:   May 9, 2003                  By:  /s/ ARTHUR T. SANDS
                                          -------------------------------------
                                          Arthur T. Sands, M.D., Ph.D.
                                          President and Chief Executive Officer


Date:   May 9, 2003                  By:  /s/ JULIA P. GREGORY
                                          -------------------------------------
                                          Julia P. Gregory
                                          Executive Vice President and
                                          Chief Financial Officer

