EXHIBIT 4.3
                      FIRST AMENDMENT TO RIGHTS AGREEMENT
                This FIRST AMENDMENT TO RIGHTS AGREEMENT (this "Amendment") is
entered into as of October 9, 2002 between Gen-Probe Incorporated, a Delaware
corporation (the "Company"), and Mellon Investor Services LLC, a New Jersey
limited liability company, as Rights Agent (the "Rights Agent"). Capitalized
terms used but not otherwise defined herein shall have the meanings ascribed to
them in the Rights Agreement (as defined below).
                WHEREAS, the Company and the Rights Agent previously entered
into the Rights Agreement dated as of September 16, 2002 between the Company and
the Rights Agent (the "Rights Agreement"); and
                WHEREAS, pursuant to Section 26 of the Rights Agreement, the
Company and the Rights Agent may from time to time supplement or amend any
provision of the Rights Agreement in accordance with the terms of such Section
26.
                NOW, THEREFORE, in consideration of the foregoing promises and
mutual agreements set forth in this Amendment, the parties hereby amend the
Rights Agreement as follows:
                1. Exhibit C (the "Summary of Rights") to the Rights Agreement
is hereby replaced in its entirety by Exhibit C attached hereto.
                2. This Amendment may be executed in one or more counterparts,
each of which when executed shall be deemed to be an original, including
counterparts transmitted by facsimile, but all of which taken together shall
constitute one and the same agreement.
                3. This Amendment shall be deemed to be a contract made under
the laws of the State of Delaware and for all purposes shall be governed by and
construed in accordance with the laws of such State applicable to contracts to
be made and performed entirely within such State.
                4. On and after the date hereof, each reference in the Rights
Agreement to the "Agreement" shall mean the Rights Agreement as amended hereby.
Except as specifically amended above, the Rights Agreement shall remain in full
force and effect and is hereby ratified and confirmed. The execution, delivery
and effectiveness of this Amendment shall not, except as expressly provided
herein, operate as a waiver of any right, power or remedy of any party hereto,
nor constitute a waiver of any provision of the Rights Agreement.
                            [SIGNATURE PAGE FOLLOWS]
                IN WITNESS WHEREOF, the parties have executed this Amendment as
of the date first written above.
                                            GEN-PROBE INCORPORATED
                                            By: /s/ R. ▇▇▇▇▇▇▇ ▇▇▇▇▇
                                                --------------------------------
                                                R. ▇▇▇▇▇▇▇ ▇▇▇▇▇
                                                Vice President and General
                                                Counsel
                                            MELLON INVESTOR SERVICES LLC,
                                            as Rights Agent
                                            By: /s/ ▇▇▇▇▇▇ ▇▇▇▇▇▇▇
                                                --------------------------------
                                                ▇▇▇▇▇▇ ▇▇▇▇▇▇▇
                                                Assistant Vice President
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                                   Exhibit C
                 SUMMARY OF RIGHTS TO PURCHASE PREFERRED SHARES
                             GEN-PROBE INCORPORATED
DISTRIBUTION OF RIGHTS:          On September 16, 2002, the Board of Directors
                                 of Gen-Probe Incorporated (the "Company")
                                 declared a dividend of one preferred share
                                 purchase right (a "Right") for each share of
                                 common stock, $.0001 par value (the "Common
                                 Shares"), of the Company outstanding at the
                                 close of business on September 26, 2002 (the
                                 "Record Date"). As long as the Rights are
                                 attached to the Common Shares, the Company will
                                 issue one Right (subject to adjustment) with
                                 each new Common Share so that all such shares
                                 will have attached Rights. The Rights are not
                                 exercisable until the Distribution Date, which
                                 is described below.
RIGHTS AGREEMENT:                The description and terms of the Rights are set
                                 forth in a Rights Agreement, dated as of
                                 September 16, 2002, as the same may be amended
                                 from time to time (the "Agreement"), between
                                 the Company and Mellon Investor Services LLC, a
                                 New Jersey limited liability company, as Rights
                                 Agent (the "Rights Agent"). Capitalized terms
                                 used herein and not otherwise defined herein
                                 shall have the meaning given to such terms in
                                 the Agreement.
TRANSFER OF RIGHTS; RIGHTS
CERTIFICATES:                    The Agreement provides that until the
                                 Distribution Date or earlier redemption,
                                 exchange, termination, or expiration of the
                                 Rights, the Rights will be evidenced, with
                                 respect to any of the Common Share certificates
                                 outstanding as of the Record Date, by such
                                 Common Share certificate together with a copy
                                 of this Summary of Rights and that the Rights
                                 will be transferred with and only with the
                                 Common Shares. Until the Distribution Date (or
                                 earlier redemption or expiration of the
                                 Rights), new Common Share certificates issued
                                 after the Close of Business on the Record Date
                                 will contain a notation incorporating the
                                 Agreement by reference. Until the Distribution
                                 Date (or earlier redemption, exchange,
                                 termination or expiration of the Rights), the
                                 surrender for transfer of any certificates for
                                 Common Shares, with or without such notation or
                                 a copy of this Summary of Rights, will also
                                 constitute the transfer of the Rights
                                 associated with the Common Shares represented
                                 by such certificate. As soon as practicable
                                 following the Distribution Date, separate
                                 certificates evidencing the Rights ("Right
                                 Certificates") will be mailed to holders of
                                 record of the Common Shares as of the Close of
                                 Business on the Distribution Date and such
                                 separate Right Certificates alone will evidence
                                 the Rights.
DISTRIBUTION DATE:               Rights will separate from the Common Shares and
                                 become exercisable upon the earlier to occur of
                                 (i) ten (10) days following a public
                                 announcement that a person or group of
                                 Affiliated or Associated persons (other than an
                                 Existing Holder (as defined below), unless and
                                 until such time as the Existing Holder becomes
                                 the beneficial owner of an additional 2% or
                                 more of the Common Shares) has acquired, or
                                 obtained the right to acquire, beneficial
                                 ownership of 15% or more of the Common Shares
                                 (an "Acquiring Person") or (ii) ten (10)
                                 business days (or such later date as may be
                                 determined by action of the Board of Directors
                                 prior to such time as any person or group of
                                 Affiliated persons becomes an
                                 Acquiring Person) following the commencement or
                                 announcement of an intention to make a tender
                                 offer or exchange offer the consummation of
                                 which would result in the beneficial ownership
                                 by a person or group of 15% or more of the
                                 Common Shares (the earlier of (i) and (ii)
                                 being called the "Distribution Date").
                                 "Existing Holder" means Capital Research and
                                 Management Company, together with all of its
                                 Affiliates and Associates. As described in the
                                 Rights Agreement, Rights which are held by or
                                 have been held by an Acquiring Person or
                                 Associates or Affiliates thereof and certain
                                 transferees thereof will become null and void
                                 and will no longer be transferable.
PREFERRED STOCK PURCHASABLE
UPON EXERCISE OF RIGHTS:         When exercisable, each Right will entitle the
                                 registered holder to purchase from the Company
                                 one one-hundredth of a share of Series A Junior
                                 Participating Preferred Stock (the "Preferred
                                 Shares") at a price of $100.00 per one
                                 one-hundredth of a Preferred Share, subject to
                                 adjustment (the "Purchase Price"), unless the
                                 "Flip-In" or "Flip-Over" provisions described
                                 below are applicable. Because of the nature of
                                 the Preferred Share's dividend, liquidation and
                                 voting rights, the value of one one-hundredth
                                 of a Preferred Share purchasable upon exercise
                                 of each Right should approximate the value of
                                 one Common Share. For more information about
                                 the Preferred Shares, see "Terms of Preferred
                                 Shares" below.
FLIP-IN:                         In the event that a Person becomes an Acquiring
                                 Person or if the Company were the surviving
                                 corporation in a merger with an Acquiring
                                 Person or any Affiliate or Associate of an
                                 Acquiring Person and the Common Shares were not
                                 changed or exchanged, each holder of a Right,
                                 other than Rights that are or were acquired or
                                 beneficially owned by the Acquiring Person
                                 (which Rights will thereafter be null and
                                 void), will thereafter have the right to
                                 receive, upon exercise, Common Shares having a
                                 market value of two times the then-current
                                 Purchase Price of the Right.
FLIP-OVER:                       In the event that, after a Person has become an
                                 Acquiring Person, the Company were acquired in
                                 a merger or other business combination
                                 transaction or more than 50% of its assets or
                                 earning power were sold, proper provision shall
                                 be made so that each holder of a Right shall
                                 thereafter have the right to receive, upon the
                                 exercise thereof at the then current Purchase
                                 Price of the Right, common stock of the
                                 acquiring company having a market value at the
                                 time of such transaction equal to two times the
                                 then current Purchase Price of the Right.
EXCHANGE PROVISION:              At any time after a Person becomes an Acquiring
                                 Person and prior to the acquisition of the
                                 Company in a merger or other business
                                 combination transaction, the sale of more than
                                 50% of the Company's assets or earning power or
                                 the acquisition by such Acquiring Person of 50%
                                 or more of the outstanding Common Shares, the
                                 Board of Directors may cause the Company to
                                 exchange the Rights (other than Rights owned by
                                 an Acquiring Person which will have become null
                                 and void), in whole or in part, for Common
                                 Shares
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                                 equal to the Spread (as defined in the
                                 Agreement), subject to adjustment.
REDEMPTION OF THE RIGHTS:        The Rights may be redeemed in whole, but not in
                                 part, at a price of $.01 per Right (the
                                 "Redemption Price") by the Board of Directors
                                 at any time prior to the time that a person
                                 becomes an Acquiring Person. The redemption of
                                 the Rights may be made effective at such time,
                                 on such basis and with such conditions as the
                                 Board of Directors in its sole discretion may
                                 establish. Immediately upon any redemption of
                                 the Rights, the right to exercise the Rights
                                 will terminate and the only right of the
                                 holders of Rights will be to receive the
                                 Redemption Price.
EXPIRATION OF THE RIGHTS:        The Rights will expire on September 26, 2012,
                                 subject to the Company's right to extend such
                                 date (the "Final Expiration Date"), unless
                                 earlier redeemed or exchanged by the Company or
                                 terminated.
AMENDMENT OF TERMS OF THE
RIGHTS:                          Any of the provisions of the Agreement may be
                                 amended by the Board of Directors of the
                                 Company for so long as the Rights are then
                                 redeemable, and after the Rights are no longer
                                 redeemable, the Company may amend or supplement
                                 the Agreement in any manner that does not
                                 adversely affect the interests of the holders
                                 of the Rights.
VOTING AND OTHER RIGHTS:         Until a Right is exercised, the holder thereof,
                                 as such, will have no rights as a stockholder
                                 of the Company beyond those as an existing
                                 stockholder, including, without limitation, the
                                 right to vote or to receive dividends.
ANTI-DILUTION PROVISIONS:        The Purchase Price payable, and the number of
                                 Preferred Shares or other securities or
                                 property issuable, upon exercise of the Rights
                                 are subject to adjustment from time to time to
                                 prevent dilution (i) in the event of a stock
                                 dividend on, or a subdivision, combination or
                                 reclassification of the Preferred Shares, (ii)
                                 upon the grant to holders of the Preferred
                                 Shares of certain rights or warrants to
                                 subscribe for or purchase Preferred Shares or
                                 convertible securities at less than the current
                                 market price of the Preferred Shares or (iii)
                                 upon the distribution to holders of the
                                 Preferred Shares of evidences of indebtedness,
                                 cash, securities or assets (excluding regular
                                 periodic cash dividends at a rate not in excess
                                 of 125% of the rate of the last regular
                                 periodic cash dividend previously paid or, in
                                 case regular periodic cash dividends have not
                                 previously been paid, at a rate not in excess
                                 of 50% of the average net income per share of
                                 the Company for the four quarters ended
                                 immediately prior to the payment of such
                                 dividend, or dividends payable in Preferred
                                 Shares (which dividends will be subject to the
                                 adjustment described in clause (i) above)) or
                                 of subscription rights or warrants (other than
                                 those referred to above). No adjustment in the
                                 Purchase Price will be required until
                                 cumulative adjustments require an adjustment of
                                 at least 1% in such Purchase Price. No
                                 fractional Preferred Shares or Common Shares
                                 will be issued (other than fractions of
                                 Preferred Shares which are integral multiples
                                 of one one-hundredth of a Preferred Share,
                                 which may, at the election of the Company, be
                                 evidenced by depository receipts), and
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                                 in lieu thereof, a payment in cash will be made
                                 based on the market price of the Preferred
                                 Shares or Common Shares on the last trading
                                 date prior to the date of exercise.
TERMS OF PREFERRED SHARES:       Each Preferred Share purchasable upon exercise
                                 of the Rights will be entitled, when, as and if
                                 declared, to a minimum preferential quarterly
                                 dividend payment of $1.00 per share but will be
                                 entitled to an aggregate dividend of 100 times
                                 the dividend, if any, declared per Common
                                 Share. In the event of liquidation, dissolution
                                 or winding up of the Company, the holders of
                                 the Preferred Shares will be entitled to a
                                 minimum preferential liquidation payment of
                                 $100 per share (plus any accrued but unpaid
                                 dividends) but will be entitled to an aggregate
                                 payment of 100 times the payment made per
                                 Common Share. Each Preferred Share will have
                                 100 votes and will vote together with the
                                 Common Shares. Finally, in the event of any
                                 merger, consolidation or other transaction in
                                 which Common Shares are exchanged, each
                                 Preferred Share will be entitled to receive 100
                                 times the amount received per Common Share.
                                 Preferred Shares will not be redeemable. These
                                 rights are protected by customary antidilution
                                 provisions.
     A copy of the Agreement has been filed with the Securities and Exchange
Commission as an Exhibit to a Current Report on Form 8-K. A copy of the
Agreement is available free of charge from the Company. This summary description
of the Rights does not purport to be complete and is qualified in its entirety
by reference to the Agreement, which is incorporated herein by reference.
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