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                                                                     EXHIBIT 5.1




                                 August 10, 2001



Lexicon Genetics Incorporated
4000 Research Forest Drive
The Woodlands, Texas 77381

Re:   Registration Statement on Form S-3 of Lexicon Genetics Incorporated

Ladies and Gentlemen:

      We have acted as counsel to Lexicon Genetics Incorporated (the "Company"),
a Delaware corporation, with respect to certain legal matters in connection with
the Company's Registration Statement on Form S-3 (the "Registration Statement")
relating to the registration by the Company under the Securities Act of 1933, as
amended (the "Securities Act"), of the offer and sale by certain stockholders of
the Company from time to time, pursuant to Rule 415 under the Securities Act, of
up to 3,527,991 shares (the "Shares") of the Company's common stock, par value
$0.001 per share.

      We have examined originals or copies, certified or otherwise identified to
our satisfaction, of the Restated Certificate of Incorporation and Restated
Bylaws of the Company, each as amended to the date hereof, and such other
certificates, documents and instruments as we considered appropriate for
purposes of the opinion hereafter expressed.

      Based on the foregoing, we are of the opinion that the Shares have been
duly authorized and validly issued and are fully paid and non-assessable.

      The foregoing opinion is limited in all respects to the laws of the State
of Delaware and the federal laws of the United States of America.

      We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to our firm under the caption "Legal
Matters" in the prospectus forming a part of the Registration Statement. By
giving such consent, we do not admit that we are within the category of persons
whose consent is required under Section 7 of the Securities Act or the rules and
regulations of the Securities and Exchange Commission issued thereunder.


                                    Very truly yours,


                                    VINSON & ELKINS L.L.P.
                                    /s/ VINSON & ELKINS L.L.P.