Document:

<PAGE>
CERTAIN MATERIAL (INDICATED BY AN ASTERISK) HAS BEEN OMITTED FROM THIS DOCUMENT
PURSUANT TO A REQUEST FOR CONFIDENTIAL TREATMENT THE OMITTED MATERIAL HAS BEEN
FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION.

                                                                   EXHIBIT 10.19
                                                                     S87-012:LRM
                                                                  Patent License
                                                                         3/26/97

                             CO-EXCLUSIVE AGREEMENT

Effective as of APRIL 23, 1997, THE BOARD OF TRUSTEES OF THE LELAND STANFORD
JUNIOR UNIVERSITY, a body having corporate powers under the laws of the State
of California, having a principal place of business at 900 Welch Road, Suite
350, Palo Alto California ("STANFORD"), and GEN-PROBE Incorporated, a
corporation organized and existing under the laws of the State of Delaware and
having a principal place of business at 9880 Campus Point Drive, San Diego,
California ("GEN-PROBE"), agree as follows:

1. BACKGROUND

1.1  STANFORD represents that it owns an assignment of all rights, titles and
     interest to the technique of Selective Amplification of Target
     Polynucleotide Sequences ("Invention"), as described in Stanford Docket
     S87-012 and pursuant to 35 U.S.C. 202, and any Licensed Patent(s), as
     hereinafter defined, which may issue to such Invention.

1.2  STANFORD has rights to certain technical data and information pertaining
     to Invention.

1.3  STANFORD wishes to have the Invention perfected and marketed at the
     earliest possible time in order that products resulting therefrom may be
     available for public use and benefit.

1.4  The Invention was made in Dr. John Boothroyd's laboratory during the course
     of research supported by the National Institutes of Health.

1.5  Gen-Probe, Inc. is a licensee of STANFORD under Licensed Patent(s) under a
     license agreement dated August 1, 1988 and an amendment thereto dated June
     19, 1993 (collectively the "Original Amended License Agreement").

1.6  GEN-PROBE and Organon Teknika B.V. desire that both GEN-PROBE and Organon
     Teknika B.V., become Co-Exclusive licensees under Licensed Patent(s).

                                      -1-
<PAGE>
1.7  STANFORD desires to grant a Co-Exclusive license to GEN-PROBE in
     recognition of:

          (a)  GEN-PROBE's agreement to allow its Original Amended License
               Agreement to be amended to allow STANFORD to grant a Co-Exclusive
               license to Organon Teknika B.V. provided that:

                    (i)  [***]

                   (ii)  [***]

                  (iii)  [***]

                   (iv)  [***]

                    (v)  [***]

          (b)  An interference settlement concerning STANFORD's US Patent No.
               5,437,990 issued August 1, 1995 and associated foreign patent
               oppositions (collectively, "The '990 Arbitration").

1.8  GEN-PROBE, for its agreement to the terms hereunder, and for its agreement
     to be bound to the terms of this Agreement regardless of the outcome of the
     '990 Arbitration wishes to acquire license under said Invention and
     Licensed Patent(s), for the purpose of undertaking development, to
     manufacture, use, and sell Licensed Products(s) in the

*** Certain information on this page has been omitted and filed separately with
the Commission. Confidential treatment has been requested with respect to the
omitted portions.

                                      -2-

<PAGE>
     Licensed Field of Use, Licensed Product(s) and Licensed Field of Use being
     terms hereinafter defined.

2. DEFINITIONS

2.1  "Licensed Patent(s)" means US Patent No. 5,437,990 issued on August 1, 1995
     and any Letters Patent issued upon STANFORD's U.S. Patent Application,
     Serial Number 080,479 filed July 31, 1987, including the information
     contained in such application, with respect to the Invention, any foreign
     patents corresponding thereto, and/or any divisions, continuations, or
     continuations-in-part, or reissue thereof.

2.2  "Licensed Product(s)" means any product or part thereof in the Licensed
     Field of Use, the manufacture, use, or sale of which:

          (a)  Is covered by valid claim of an issued, unexpired Licensed
               Patent(s) directed to the Invention. A claim of an issued,
               unexpired Licensed Patent(s) shall be presumed to be valid unless
               and until it has been held to be invalid by a final judgment of a
               court of competent jurisdiction from which no appeal can be or is
               taken;

          (b)  Is covered by any claim being prosecuted in a pending
               application directed to the Invention.

     For the avoidance of doubt, GEN-PROBE herewith confirms that in the event a
     Siska patent (i.e., a patent issuing from U.S. application Nos.
     07/845,739,07/202,978, or 07/064,141 or any application related thereto)
     prevails in the '990 Arbitration GEN-PROBE shall continue to pay royalties,
     as per Paragraph 6.2, on any product that would otherwise be included in
     the definition of Licensed Product(s) under this Paragraph 2.2 without
     regard to the outcome of the '990 Arbitration.

2.3  "Net Sales" means the gross revenue of the Licensed Product(s) in the form
     in which it is sold or used, whether or not assembled (and without
     excluding therefrom any components or subassemblies thereof, whatever their
     origin and whether or not patent impacted), less the following items but
     only insofar as they actually pertain to the

                                      -3-

<PAGE>
     disposition of such Licensed Product(s) by GEN-PROBE and/or sublicensee(s)
     and are included in such gross income, and (except Items (d) and (e)) are
     separately billed:

          (a)  Import, export, excise, and sales taxes, plus custom duties;

          (b)  Cost of insurance, from the place of manufacture or point of
               installation, packing, and transportation to the customer's
               premises or point of installation;

          (c)  Costs of installation at the place of use;

          (d)  Credit for returns, rejections, recalls, allowances, or trades;
               and

          (e)  a reasonable allocation for the rental of an instrument when the
               product is sold under a reagent rental program, except where the
               instrument is a Licensed Product.

2.3.1  In the case where GEN-PROBE sells a Combination Product, prior to
       calculating the royalty under Section 6.2, Net Sales of the Combination
       Product shall be multiplied by the formula A/B where A is the gross
       selling price of a Licensed Product sold separately which performs a
       function of the Combination Product and B is the gross selling price of
       the Combination Product or, if the Licensed Product is not sold
       separately, by the formula C/C+D where C is the fully allocated cost of
       the components of the Combination Product which would constitute a
       Licensed Product and D is the fully allocated cost of the components of
       the Combination Product which perform a function which if performed
       separately would not constitute a Licensed Product.

2.4  "Combination product" means a product which performs more than one
     function, for example, detection of two different organisms or both
     detection of an infectious organism and its resistance to antibiotics, one
     function of which if packaged as a separate product would be a Licensed
     Product and another function of which if packaged as a separate product
     would not be a Licensed Product, i.e., not covered by a claim in

                                      -4-

<PAGE>
     an issued patent or pending patent application licensed under this
     Agreement.

2.5  "Licensed Field of Use" means all fields of use (including, without
     limitation, [***]), excluding:

          (a)  [***]; and

          (b)  [***]

2.6  "Licensed Territory" means the world.

2.7  "Co-Exclusive" means that, subject to Article 4, no other licenses will be
     granted to any party in the Licensed Territory in the Licensed Field other
     than Organon Teknika and GEN-PROBE.

2.8  "Affiliate" means any entity which by means of ownership of a majority of
     voting shares or at least 50% interest in income or otherwise directly or
     indirectly controls, is controlled by or is under common control with
     either party.

3. NOVATION

STANFORD and GEN-PROBE agree that the certain Original Amended License
Agreement is hereby terminated and superseded in its entirety by this License
Agreement. Both parties agree that no further rights or obligations survive
from such superseded certain Original Amended License Agreement.

4. GRANT

4.1  STANFORD hereby grants and GEN-PROBE hereby accepts a license in the
     Licensed Field of Use to make, have made, use, and sell Licensed
     Products(s) in the Licensed Territory.

*** Certain information on this page has been omitted and filed separately with
the Commission. Confidential treatment has been requested with respect to the
omitted portions.

                                     - 5 -
<PAGE>
4.2  Said license shall be Co-Exclusive in the Licensed Field of Use for the
     life of the last to expire of Licensed Patent(s).

4.3  GEN-PROBE shall have the right to grant sublicenses to third parties to
     make, have made, use and sell Licensed Product(s) in the Licensed
     Territory.

     4.3.1  Such sublicenses are subject to the restriction that any such
            sublicense shall not be on any terms less favorable to STANFORD than
            the terms of this Agreement and that any sublicensee hereunder shall
            not be permitted to further sublicense.

     4.3.2  Any such sublicense shall also expressly include the provisions of
            Articles 7, 8, and 9 for the benefit of STANFORD.

     4.3.3  Upon termination of this Agreement for any cause, any sublicense
            granted hereunder shall continue with STANFORD provided the
            sublicensee agrees to thereafter assume the obligations of GEN-PROBE
            insofar as they correspond to the scope of the sublicense.

5. GOVERNMENT RIGHTS

This Agreement is subject to all of the terms and conditions of Public Law
96-517 as amended, and GEN-PROBE agrees to take all action necessary on its
part as licensee to enable STANFORD to satisfy its obligation thereunder,
relating to Invention.

6. ROYALTIES

6.1  GEN-PROBE has paid and STANFORD has received license issue fees under the
     Original Amended License Agreement and no further license issue fees are
     due hereunder.

6.2  GEN-PROBE shall pay STANFORD earned royalties on the sale of Licensed
     Product(s) of (i) [***] on GEN-PROBE's Net Sales; and (ii)
     [***] on GEN-PROBE's sublicensee's Net Sales.

*** Certain information on this page has been omitted and filed separately with
the Commission. Confidential treatment has been requested with respect to the
omitted portions.

                                     - 6 -
<PAGE>
6.3  Beginning on April 1, 1998, and each April 1 thereafter during the term of
     this Agreement, GEN-PROBE shall pay STANFORD a minimum annual royalty of
     [***]. Said minimum annual royalty payments are nonrefundable, but they are
     creditable to the extent provided in Paragraph 6.4 below.

6.4  Creditable payments under this Agreement pursuant to Paragraph 6.1 and 6.3
     shall be an offset to GEN-PROBE against up to one hundred percent (100%) of
     each earned royalty payment which GEN-PROBE would be required to pay
     pursuant to Paragraph 6.2 until the entire credit is exhausted. Credits of
     [***] accumulated by GEN-PROBE under the Original Amended License Agreement
     will carry forward under this Agreement.

6.5  If this Agreement is not terminated in accordance with other provisions
     hereof, GEN-PROBE's obligation to pay royalties hereunder shall continue
     for so long as GEN-PROBE, by its activities would, but for the license
     granted herein, infringe a valid claim of an unexpired Licensed Patent(s)
     of STANFORD, or practice a valid claim issuing from U.S. application Nos.
     07/845,739, 07/202,978, or 07/064,141, covering said activity which has
     prevailed in the '990 Arbitration over a claim of a Licensed Patent(s).

6.6  The royalty on sales in currencies other than U.S. Dollars shall be
     calculated using the appropriate foreign exchange rate for such currency
     quoted by the Bank of America (San Francisco) foreign exchange desk, on the
     close of business on the last banking day of each calendar quarter. Royalty
     and payments to STANFORD shall be in U.S. Dollars and shall be net of all
     non-U.S. taxes.

6.7  Within thirty (30) days after receipt of a statement from STANFORD,
     GEN-PROBE shall reimburse STANFORD for one-half of the costs incurred after
     the Effective Date by STANFORD in connection with the filing and
     prosecution of all patent applications and maintenance of

                                      -7-

*** Certain information on this page has been omitted and filed separately with
the Commission. Confidential treatment has been requested with respect to the
omitted portions.
<PAGE>
     patents corresponding to the Invention, and these costs shall be paid by
     GEN-PROBE.

7. REPORTS, PAYMENTS, AND ACCOUNTING

7.1  Quarterly Royalty Payment and Report. GEN-PROBE shall make written reports
     and royalty payments to STANFORD within ninety (90) days after the end of
     each calendar quarter. This report shall state the number, description, and
     aggregate Net Sales of Licensed Product(s) during such completed calendar
     quarter, and resulting calculation pursuant to Paragraph 6.2 of earned
     royalty payment due STANFORD for such completed calendar quarter.
     Concurrent with the making of each such report, GEN-PROBE shall include
     payment due STANFORD of royalties for the calendar quarter covered by such
     report.

7.2  Accounting. GEN-PROBE agrees to keep records for a period of three (3)
     years showing the manufacturing, sales, use, and other disposition of
     products sold or otherwise disposed of under the license herein granted in
     sufficient detail to enable the royalties payable hereunder by GEN-PROBE to
     be determined, and further agrees to permit its books and records to be
     examined to the extent necessary to verify reports provided for in
     Paragraph 7.1 by an independent certified public accountant, provided that
     such audits occur no more than one (1) time per calendar year and provided
     further that accountants shall report to STANFORD only errors regarding
     calculation of royalties. Such examination is to be made by STANFORD, at
     the expense of STANFORD, except in the event that the results of the audit
     reveal a discrepancy in GEN-PROBE'S favor of ten percent (10%) or more,
     then the audit fees shall be paid by GEN-PROBE.

8. WARRANTY AND NEGATION OF WARRANTIES

8.1  Nothing in this Agreement is or shall be construed as:

                                      -8-

<PAGE>
          (a)  A warranty or representation by STANFORD as to the validity or
               scope of any Licensed Patent(s);

          (b)  A warranty or representation that anything made, used, sold, or
               otherwise disposed of under any license granted in this
               Agreement is or will be free from of patents, copyrights, and
               other rights of third parties;

          (c)  An obligation to bring or prosecute actions or suits against
               third parties for infringement, except to the extent and in the
               circumstances described in Article 12; or

          (d)  Granting by implication, estoppel, or otherwise any licenses
               under patents of STANFORD or other persons other than Licensed
               Patent(s), regardless of whether such patents are dominant or
               subordinate to any Licensed Patent(s).

8.2  EXCEPT AS EXPRESSLY SET FORTH IN THIS AGREEMENT, STANFORD MAKES NO
     REPRESENTATIONS AND EXTENDS NO WARRANTIES OF ANY KIND, EITHER EXPRESS
     OR IMPLIED. THERE ARE NO EXPRESS OR IMPLIED WARRANTIES OF MERCHANTABILITY
     OR FITNESS FOR A PARTICULAR PURPOSE, OR THAT THE USE OF THE LICENSED
     PRODUCTS(S) WILL NOT INFRINGE ANY PATENT, COPYRIGHT, TRADEMARK, OR OTHER
     RIGHTS OF THIRD PARTIES.

9. INDEMNITY

9.1  GEN-PROBE agrees to indemnify, hold harmless, and defend STANFORD and its
     trustees, officers, employees, students, and agents against any and all
     claims for death, illness, personal injury, property damage, and improper
     business practices arising out of the manufacture, use, sale, or other
     disposition of Invention, Licensed Patent(s), or Licensed Product(s) by
     GEN-PROBE, its sublicensees or their customers.

                                      -9-
<PAGE>
9.2  GEN-PROBE shall at all times comply, through insurance or self-insurance,
     with all statutory workers' compensation and employers' liability
     requirements covering any and all employees with respect to activities
     performed under this Agreement.

9.3  GEN-PROBE shall maintain, beginning on the first day GEN-PROBE or any
     GEN-PROBE sublicensee ships a therapeutics product manufactured by the
     licensed process, commercial general liability insurance, including
     products liability insurance, with minimum limits of liability of $5
     million with reputable and financially secure insurance carrier(s) to cover
     the activities of GEN-PROBE and its sublicensee(s). Any and all such
     policies of insurance described in the previous sentence shall include as
     additional named insureds STANFORD, Stanford Health Services, their
     trustees, directors, officers, employees, students and agents, and shall
     provide that such policies may not, without 30 days prior written notice to
     STANFORD, be cancelled or changed to materially adversely affect any such
     additional named insured's coverage. Such insurance shall be written to
     cover liability of such additional named insureds incurred beginning on the
     first day GEN-PROBE or any GEN-PROBE sublicensee ships a therapeutics
     product manufactured by the licensed process and continuing until five
     years after the term of this Agreement. The parties agree that the coverage
     of GEN-PROBE's liability insurance policy(ies) shall be primary, and that
     the coverage of any STANFORD or Stanford Health Services liability
     insurance policy(ies) shall be excess, with respect to liability
     aggregating up to $5,000,000, and that GEN-PROBE shall cause its liability
     insurance contract(s) to so provide. Upon STANFORD's request, GEN-PROBE
     shall from time to time furnish to STANFORD a certificate listing
     GEN-PROBE's applicable insurance policies and confirming compliance with
     all provisions of this Paragraph.

                                      -10-
<PAGE>
10. MARKING

GEN-PROBE agrees to mark Licensed Product(s) made, sold, or otherwise disposed
of by it under the license granted in this Agreement with the numbers of the
Licensed Patent(s) on the Licensed Products(s)' inserts.

11. PROMOTIONAL ADVERTISING

GEN-PROBE agrees not to identify STANFORD in any promotional advertising or
other promotional materials to be disseminated to the public or any portion
thereof or to use the name of any STANFORD faculty member, employee, or student
or any trademark, service mark, trade name, or symbol of STANFORD or Stanford
Health Services, or that is associated with either of them, without STANFORD'S
prior written consent, except where GEN-PROBE is so required to disclose such
relationship by law, regulation or ordinance.

12. INFRINGEMENT BY OTHERS; PROTECTION OF PATENTS

12.1 STANFORD AND GEN-PROBE shall promptly inform the other of any suspected
     infringement of any Licensed Patent(s) by a third party.

12.2 GEN-PROBE agrees to meet with STANFORD and Organon Teknika B.V. to discuss
     a mutually acceptable course of action prior to commencing any legal
     proceeding against any suspected infringer.

13. COMMERCIAL APPLICATION

As an inducement to STANFORD to enter into this Agreement, GEN-PROBE agrees to
use all reasonable efforts and diligence to proceed with the development,
manufacture, and sale or lease of Licensed Product(s) and to diligently develop
markets for the Licensed Product(s). Notwithstanding the foregoing, nothing
herein shall be construed or interpreted as a limitation on the right of
GEN-PROBE, without being in breach of this Agreement, to improve and/or
reconfigure its products, including any Licensed Products(s),

                                      -11-
<PAGE>
to make use of new or different technology, even though doing so might mean
that such products would not (or no longer) be Licensed Product(s) under this
Agreement.

14. TERMINATION

14.1  GEN-PROBE may terminate this Agreement only if it is not practicing
      Licensed Patent(s) or patents issuing from U.S. application Nos.
      07/845,739,07/202,978, or 07/064/141 (except as limited by Paragraph 14.3)
      by giving STANFORD notice in writing at least thirty (30) days in advance
      of the effective date of termination selected by GEN-PROBE.

14.2  STANFORD may terminate this Agreement (except as limited by Paragraph
      14.3) if GEN-PROBE:

          (a)  Is in material default in payment of royalty or providing of
               reports; or

          (b)  Is in breach of any material provision hereof; or

          (c)  Provides any materially false report; and

      GEN-PROBE fails to remedy any such default, breach, or false report within
      sixty (60) days after written notice hereof by STANFORD.

14.3  Surviving any termination are:

          (a)  GEN-PROBE's obligation to pay royalties accrued or accruable;

          (b)  Any cause of action or claim of GEN-PROBE or STANFORD, accrued or
               to accrue, because of any breach or default by the other party;
               and

          (c)  The provisions or Articles 7, 8, and 9.

14.4  Force Majeure. No liability hereunder shall result to either Party from
      delay in performance caused by force majeure, that is, circumstances
      beyond the reasonable control of the party affected thereby, including,
      without limitation, Acts of God, fire, flood, war, government regulations,
      labor unrest, or shortage of or inability to obtain material or equipment.

                                      -12-
<PAGE>
15.ASSIGNMENT

This Agreement shall not be assignable by GEN-PROBE except to an Affiliate or
to the successor of all or substantially all of its business and assets to
which this Agreement relates resulting from transfer by sale, merger or
consolidation of such business and assets or upon the written consent of
STANFORD.

16.ARBITRATION

16.1  Any controversy arising under or related to this Agreement, or any
      disputed claim by either party against the other under this Agreement
      excluding any dispute relating to patent validity or infringement arising
      under this Agreement, shall upon request of either party be settled by
      arbitration in accordance with the Licensing Agreement Arbitration Rules
      of the American Arbitration Association. Upon request of either party,
      arbitration will be by:

      (a) A third party arbitrator mutually agreed upon in writing by GEN-PROBE
          and STANFORD within thirty (30) days of such arbitration request; or

      (b) A member of the American Arbitration Association.

      Judgment upon the award rendered by the Arbitrator may be entered in any
      court having jurisdiction thereof.

16.2  The parties shall be entitled to discovery in like manner as if the
      arbitration were a civil suit in the California Superior Court; provided,
      however, the arbitrator shall have the full authority to limit the scope
      and amount of discovery and pleadings as he deems proper in order to meet
      the time constraints set forth in Paragraph 16.4 below, and the parties
      agree to cooperate toward such end.

16.3  Any arbitration shall be held at Stanford, California, unless the parties
      hereto mutually agree in writing to another place.

                                      -13-

<PAGE>
16.4 Unless otherwise mutually agreed, any arbitrator chosen hereunder shall be
     a duly licensed patent attorney having experience with biotechnology
     patents and having patent licensing experience, who has not (and whose firm
     has not) represented either party. As a condition to accepting and
     presiding over the arbitration, the arbitrator shall agree to conclude the
     proceedings within 6 months after being named as arbitrator and to render a
     written decision within 45 days after conclusion of the arbitration
     hearing.

16.5 Notwithstanding any preceding portion of this Article 16, no demand for
     arbitration shall be made and no arbitration shall be valid if the time for
     bringing the underlying claim has expired under the applicable statute of
     limitation as determined under California law.

17.NOTICES

All notices under this Agreement shall be deemed to have been fully given when
done in writing and deposited in the United States mail, as follows:

               TO STANFORD:   Office of Technology Licensing
                              Stanford University
                              900 Welch Road, Suite 350
                              Palo Alto, CA 94304-1850

               Attention:     Director

               TO GEN-PROBE:  Gen-Probe, Inc.
                              9880 Campus Point Drive
                              San Diego, CA 92121-1589

               Attention:     President

Either party may change its address upon written notice to the other party.

17.ENTIRE AGREEMENT

This Agreement constitutes the entire agreement and understanding between the
parties and supersedes all prior agreements and understandings with respect to
Licensed Product(s) whether written or oral. No modification or

                                      -14-

<PAGE>
claimed waiver of any of the provisions hereof shall be valid unless in writing
and signed by authorized representatives of the party against whom such
modification or waiver is sought to be enforced.

18. APPLICABLE LAW

This Agreement shall be construed, interpreted, and applied in accordance with
the laws of the State of California.

19. OTHER

19.1  The headings of Articles and Paragraphs in this Agreement are inserted for
      reference only and shall not be deemed a part of the text hereof.

19.2  In the event that any provision of this Agreement shall be determined to
      be unenforceable, all other provisions shall remain in full force and
      effect.

IN WITNESS WHEREOF, the parties hereto have executed Agreement in duplicate
originals by their duly authorized officers or representatives.

GEN-PROBE INC.                          THE BOARD OF TRUSTEES OF THE
                                        LELAND STANFORD JUNIOR
                                        UNIVERSITY

By      /s/ Henry L. Nordhoff            By    /s/
   -------------------------------        -------------------------------------
Title  CEO                              Title  Acting Director
     -----------------------------           ----------------------------------
Date  23-April-97                       Date  April 21, 1997
    ------------------------------          -----------------------------------

[APPROVED LEGAL SEAL]

                                      -15-<PAGE>
CERTAIN MATERIAL (INDICATED BY AN ASTERISK) HAS BEEN OMITTED FROM THIS DOCUMENT
PURSUANT TO A REQUEST FOR CONFIDENTIAL TREATMENTS. THE OMITTED MATERIAL HAS
BEEN FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION.

                                                                   Exhibit 10.20

                                AMENDMENT NO. 1
                                     to the
                               License Agreement
                            Effective April 23, 1997
                                    Between
                              STANFORD UNIVERSITY
                                      and
                             GEN-PROBE INCORPORATED

          Effective April __, 1998 THE BOARD OF TRUSTEES OF THE LELAND STANFORD
JUNIOR UNIVERSITY, having a principal place of business at 900 Welch Road,
Suite 350, Palo Alto, CA 94304 (hereinafter "STANFORD") and GEN-PROBE
INCORPORATED, having a principal place of business at 10210 Genetic Center
Drive, San Diego, CA 92121 (hereinafter "GEN-PROBE"), agree as follows:

                                 1. BACKGROUND

1.1  STANFORD and GEN-PROBE are parties to a License Agreement effective April
     23, 1997 ("Original Agreement") concerning the invention entitled
     "Selective Amplification of Target Polynucleotide Sequences" ("Invention")
     as described in Stanford Docket S87-012.

1.2  STANFORD and GEN-PROBE wish to amend the Original Agreement so that
     GEN-PROBE may practice the Invention in a broadened [***] field of use.

                                 2. AMENDMENTS

2.1  Paragraph 1.8 of the Original Agreement is amended by replacing the words
     "in the Licensed Field of Use, Licensed Product(s), and Licensed Field of
     Use being terms" with the following:

          "in the Licensed Field of Use and the Nonexclusive Field of Use,
          Licensed Product(s), Licensed Field of Use, and Nonexclusive Field of
          Use being terms".

2.2  Paragraph 2.2 of the Original Agreement is amended by inserting in the
     second line the phrase "and the Nonexclusive Field of Use" immediately
     after the phase "Licensed Field of Use".

2.3  Add a new Paragraph "2.9" to the Original Agreement as follows:

          "2.9  "Nonexclusive Field of Use" means [***}.

2.4  Add a new Paragraph "4.4" to the Original Agreement as follows:

          "4.4  STANFORD hereby grants and GEN-PROBE hereby accepts a
                nonexclusive license in the Nonexclusive Field of Use to make,
                have made, use, and sell Licensed Product(s) in the Licensed
                Territory for the life of the last to expire of Licensed
                Patent(s)."

*** Certain information on this page has been omitted and filed separately with
the Commission. Confidential treatment has been requested with respect to the
omitted portions.
<PAGE>
     2.5  All other provisions of the Original Agreement remain in full force
          and effect.

          IN WITNESS WHEREOF, the parties hereto have executed this Amendment
          No. 1 in duplicate originals.

               THE BOARD OF TRUSTEES OF THE LELAND
               STANFORD JUNIOR UNIVERSITY

               By /s/ Katharine Ku
                 --------------------------------------------
                   Director, Office of Technology Licensing

               Date   May 4, 1998
                   -------------------------

               GEN-PROBE INCORPORATED

               By /s/ Henry L. Nordhoff
                 ------------------------------------------
                    Henry L. Nordhoff

               Title President and Chief Executive Officer
                    ---------------------------------------

               Date   April 24, 1998
                   --------------------------

LEGAL DEPT. APPROVAL SEAL

                                       2

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