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                                                                    EXHIBIT 32.1

                                  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
                  September 30, 2003, and to which this Certification is
                  attached as Exhibit 32.1 (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 6th day of November, 2003.

Date: November 6, 2003             By: /s/ ARTHUR T. SANDS
                                      ------------------------------------------
                                       Arthur T. Sands, M.D., Ph.D.
                                       President and Chief Executive Officer

Date: November 6, 2003             By: /s/ JULIA P. GREGORY
                                      ------------------------------------------
                                       Julia P. Gregory
                                       Executive Vice President and
                                       Chief Financial Officer

