Share Option Agreement - NeXstar Pharmaceuticals Inc. and Gilead Sciences Inc.


                                SHARE OPTION AGREEMENT

     THIS SHARE OPTION AGREEMENT (the 'Option Agreement') is entered into as of
February 28, 1999, by and between NEXSTAR PHARMACEUTICALS, INC., a Delaware
corporation (the 'Company'), and GILEAD SCIENCES, INC., a Delaware corporation
(the 'Grantee').

                                       RECITALS

     A.   The Grantee, Gazelle Acquisition Sub, Inc., a Delaware corporation and
a wholly owned subsidiary of the Grantee ('Merger Sub'), and the Company are
entering into an Agreement and Plan of Merger of even date herewith (as amended
from time to time, the 'Merger Agreement'), which provides (subject to the
conditions set forth therein) for the merger of Merger Sub into the Company (the
'Merger').

     B.   As a condition to the willingness of the Grantee to enter into the
Merger Agreement, the Grantee has required that the Company enter into, and in
order to induce the Grantee to enter into the Merger Agreement, the Company
desires to enter into, this Option Agreement.

                                      AGREEMENT

     The parties to this Option Agreement, intending to be legally bound, agree
as follows:

     1.   CERTAIN DEFINITIONS.  Capitalized terms used but not defined in this
Option Agreement shall have the meanings ascribed to such terms in the Merger
Agreement.

     2.   GRANT OF OPTION.  The Company hereby grants to the Grantee an
irrevocable option (the 'Option') to purchase, out of the authorized but
unissued Company Common Stock, a number of shares of Company Common Stock equal
to up to 19.9% of the shares of Company Common Stock outstanding as of the date
hereof (as adjusted as set forth herein, the 'Option Shares'), at a price per
Option Share equal to the Exercise Price.  For purposes of this Option
Agreement, the 'Exercise Price' shall be equal to $17.48 (subject to adjustment
as set forth herein).

     3.   TERM.  The Option shall terminate on the earliest of the following
dates (the 'Termination Date'): (a) the date on which the Merger becomes
effective; (b) the first anniversary of the date on which the Grantee receives
written notice from the Company of the occurrence of an Exercise Event (as
defined in Section 4(b)); or (c) the date on which the Merger Agreement is
validly terminated pursuant to Section 6.1 thereof, if an Exercise Event shall
not have occurred on or prior to such date; PROVIDED, HOWEVER, that with respect
to clause (b) of this


                                           


sentence, if the Option cannot be exercised on such first anniversary by reason
of any applicable law, regulation, order, judgment, decree or other legal
impediment, then the Termination Date shall be extended until the date 30 days
after the date on which such impediment is removed.  The rights and obligations
set forth in Sections 8 and 9 shall not terminate on the Termination Date, but
shall extend to such time as is provided in those Sections.

     4.   EXERCISE OF OPTION.

          (a)  The Grantee may exercise the Option, in whole or in part, at any
time and from time to time on or before the Termination Date following the
occurrence of an Exercise Event (as defined in Section 4(b)).  Notwithstanding
the occurrence of the Termination Date, the Grantee shall be entitled to
purchase those Option Shares with respect to which it has exercised the Option
in accordance with the terms hereof prior to the Termination Date.

          (b)  As used herein, an 'Exercise Event' shall be deemed to have
occurred if the Grantee shall have the right to terminate the Merger Agreement:

               (i)  pursuant to Section 6.1(c) thereof and a Takeover Proposal
     shall have been previously disclosed, announced, commenced, submitted or
     made; or

               (ii) pursuant to Section 6.1(g) or 6.1(h) thereof.

          (c)  In the event the Grantee wishes to exercise the Option with
respect to any Option Shares, the Grantee shall send to the Company a written
notice (the date of which being herein referred to as the 'Notice Date')
specifying: (i) the number of Option Shares the Grantee will purchase; (ii) the
place at which such Option Shares are to be purchased; and (iii) the date on
which such Option Shares are to be purchased, which shall not be earlier than
two business days nor later than twenty business days after the Notice Date. 
The closing of the purchase of such Option Shares (the 'Closing') shall take
place at the place specified in such written notice and on the date specified in
such written notice (the 'Closing Date'); PROVIDED, HOWEVER, that: (A) if such
purchase cannot be consummated by reason of any applicable law, regulation,
order, judgment, decree or other legal impediment, the Closing Date may be
extended by the Grantee to a date not more than 30 days after the date on which
such impediment is removed; and (B) if prior notification to or approval of any
governmental authority is required (or if any waiting period must expire or be
terminated) in connection with such purchase, the Company shall promptly cause
to be filed the required notice or application for approval and shall
expeditiously process the same (and the Company shall cooperate with the Grantee
in the filing of any such notice or application required to be filed by the
Grantee and the obtaining of any such


                                         -2-


approval required to be obtained by the Grantee), and the Closing Date may be
extended by the Grantee to a date not more than 30 days after the date on which
any required notification has been made, approval has been obtained or waiting
period has expired or been terminated. 

          (d)  Notwithstanding Section 4(c), so long as the Company shall have
fully complied with all of its obligations under this Option Agreement, in no
event shall any Closing Date be more than 12 months after the related Notice
Date, and, if the Closing Date shall not have occurred within 12 months after
the related Notice Date, the exercise of the Option effected on the Notice Date
shall be deemed to have expired.  

     5.   PAYMENT AND DELIVERY OF CERTIFICATES.

          (a)  On each Closing Date, the Grantee shall pay to the Company in
immediately available funds by wire transfer to a bank account designated by the
Company an amount equal to the Exercise Price multiplied by the number of Option
Shares to be purchased on such Closing Date.

          (b)  At each Closing, simultaneously with the delivery of immediately
available funds as provided in Section 5(a), the Company shall deliver to the
Grantee a certificate or certificates representing the Option Shares to be
purchased at such Closing, which Option Shares shall be duly authorized, validly
issued, fully paid and nonassessable and free and clear of all liens, security
interests, charges or other encumbrances ('Encumbrances').

          (c)  Certificates for the Option Shares delivered at each Closing
shall be endorsed with a restrictive legend that shall read substantially as
follows:

          THE TRANSFER OF THE STOCK REPRESENTED BY THIS CERTIFICATE IS SUBJECT
          TO RESTRICTIONS ARISING UNDER THE SECURITIES ACT OF 1933, AS AMENDED,
          AND PURSUANT TO THE TERMS OF A SHARE OPTION AGREEMENT DATED AS OF
          FEBRUARY 28, 1999.  A COPY OF SUCH AGREEMENT WILL BE PROVIDED TO THE
          HOLDER HEREOF WITHOUT CHARGE UPON RECEIPT BY THE COMPANY OF A WRITTEN
          REQUEST THEREFOR.

A new certificate or certificates evidencing the same number of shares of the
Company Common Stock will be issued to the Grantee in lieu of the certificate
bearing the above legend, and such new certificate shall not bear such legend,
insofar as it applies to the Securities Act, if the Grantee shall have delivered
to the Company a copy of a letter from the staff of the Commission, or an
opinion of counsel in form and substance reasonably satisfactory to the Company
and its counsel, to the effect that such legend is not required for purposes of
the Securities Act.


                                         -3-


     6.   ADJUSTMENT UPON CHANGES IN CAPITALIZATION, ETC.

          (a)  In the event of any change in the Company Common Stock by reason
of a stock divided, split-up, combination, recapitalization, exchange of shares
or similar transaction, the type and number of shares or securities subject to
the Option, and the Exercise Price therefor, shall be adjusted appropriately,
and proper provision shall be made in the agreements governing such transaction,
so that the Grantee shall receive upon exercise of the Option the same class and
number of outstanding shares or other securities or property that Grantee would
have received in respect of the Company Common Stock if the Option had been
exercised immediately prior to such event or the record date therefor, as
applicable.  If any additional shares of Company Common Stock are issued after
the date of this Option Agreement (other than pursuant to an event described in
the first sentence of this Section 6(a)), the number of shares of Company Common
Stock then remaining subject to the Option shall be adjusted so that, after such
issuance of additional shares, such number of shares then remaining subject to
the Option, together with any shares theretofore issued pursuant to the Option,
equals 19.9% of the number of shares of the Company Common Stock then issued and
outstanding.

          (b)  If the Company shall enter into an agreement (i) to consolidate,
exchange, shares or merge with any Person, other than the Grantee or one of the
Grantee's subsidiaries, and, in the case of a merger, shall not be the
continuing or surviving corporation, (ii) to permit any Person, other than the
Grantee or one of the Grantee's subsidiaries, to merge into the Company and the
Company shall be the continuing or surviving corporation, but, in connection
with such merger, the then outstanding shares of Company Common Stock shall be
changed into or exchanged for stock or other securities of the Company or any
other Person or cash or any other property, or the shares of Company Common
Stock outstanding immediately before such merger shall after such merger
represent less than 50% of the common shares and common share equivalents of the
Company outstanding immediately after the merger, or (iii) to sell, lease or
otherwise transfer all or substantially all of its assets to any Person, other
than the Grantee or one of the Grantee's subsidiaries, then, and in each such
case, proper provision shall be made in the agreement governing such
transactions so that the Option shall, upon the consummation of any such
transaction and upon the terms and conditions set forth herein, become
exercisable for the stock, securities, cash or other property that would have
been received by the Grantee if the Grantee had exercised this Option
immediately prior to such transaction or the record date for determining the
stockholders entitled to participate therein, as appropriate.

          (c)  The provisions of Sections 7, 8 and 9 shall apply with
appropriate adjustments to any securities for which the Option becomes
exercisable pursuant to this Section 6.


                                         -4-


     7.   REPURCHASE AT THE OPTION OF GRANTEE.

          (a)  At any time on or prior to the Termination Date, at the request
of the Grantee made at any time after the first Exercise Event and ending on the
first anniversary thereof (the 'Put Period'), the Company (or any successor
thereto) shall repurchase from the Grantee (i) that portion of the Option that
then remains unexercised and (ii) all (but not less than all) of the shares of
Company Common Stock purchased by the Grantee pursuant hereto and with respect
to which the Grantee then has beneficial ownership.  The date on which the
Grantee exercises its rights under this Section 7 is referred to as the 'Grantee
Request Date.' Such repurchase shall be at an aggregate price (the 'Section 7
Repurchase Consideration') equal to the sum of:

               (i)  the aggregate exercise price paid (or, in the case of Option
     Shares with respect to which the Option has been exercised but the Closing
     Date has not occurred, payable) by the Grantee for any Option Shares as to
     which the Option has theretofore been exercised and with respect to which
     the Grantee then has beneficial ownership or has exercised the right to
     acquire beneficial ownership;

               (ii) the excess, if any, of the Applicable Price (as defined in
     Section 7(c)), over the Exercise Price (subject to adjustment pursuant to
     Section 6) paid (or, in the case of Option Shares with respect to which the
     Option has been exercised but the Closing Date has not occurred, payable)
     by the Grantee for each Option Share as to which the Option has been
     exercised and with respect to which the Grantee then has beneficial
     ownership, multiplied by the number of such shares; and

               (iii) the excess, if any, of (x) the Applicable Price for each
     share of Company Common Stock over (y) the Exercise Price (subject to
     adjustment pursuant to Section 6), multiplied by the number of Option
     Shares as to which the Option has not been exercised.

          (b)  If the Grantee exercises its rights under this Section 7, the
Company shall, within five business days after the Grantee Request Date, pay the
Section 7 Repurchase Consideration to the Grantee in immediately available
funds, and the Grantee shall surrender to the Company the Option and the
certificates evidencing the shares of Company Common Stock purchased thereunder
with respect to which the Grantee then has beneficial ownership, and the Grantee
shall warrant to the Company that, immediately prior to the repurchase thereof
pursuant to this Section 7, the Grantee had sole record and beneficial ownership
of such shares and that such shares were then held free and clear of all
Encumbrances.

          (c)  For purposes of this Option Agreement, the 'Applicable Price'
means the highest of (i) the highest purchase


                                         -5-


price per share paid pursuant a tender or exchange offer made for shares of
Company Common Stock after the date hereof and on or prior to the Grantee
Request Date, (ii) the price per share to be paid by any third Person for shares
of Company Common Stock pursuant to an agreement for a merger or other business
combination transaction with the Company entered into on or prior to the Grantee
Request Date, or (iii) the highest bid price per share of Company Common Stock
as quoted on The Nasdaq National Market (or if Company Common Stock is not
quoted on The Nasdaq National Market, the highest bid price per share as quoted
on any other market comprising a part of The Nasdaq Stock Market or, if the
shares of Company Common Stock are not quoted thereon, on the principal trading
market (as defined in Regulation M under the Exchange Act) on which such shares
are traded as reported by a recognized source) during the 60 business days
preceding the Grantee Request Date.  If the consideration to be offered, paid or
received pursuant to either of the foregoing clauses (i) or (ii) shall be other
than in cash, the value of such consideration shall be determined in good faith
by an independent nationally recognized investment banking firm selected by the
Grantee and reasonably acceptable to the Company, which determination shall be
conclusive for all purposes of this Option Agreement.

     8.   REGISTRATION RIGHTS.

          (a)  The Company shall, if requested by the Grantee at any time and
from time to time within two years of the first exercise of the Option (the
'Registration Period'), as expeditiously as practicable, prepare, file and cause
to be made effective up to two registration statements under the Securities Act
if such registration is necessary or desirable in order to permit the offering,
sale and delivery of any or all shares of Company Common Stock or other
securities that have been acquired by or are issuable to the Grantee upon
exercise of the Option in accordance with the intended method of sale or other
disposition stated by the Grantee, including, at the sole discretion of the
Company, a 'shelf' registration statement under Rule 415 under the Securities
Act or any successor provision, and the Company shall use all reasonable efforts
to qualify such shares or other securities under any applicable state securities
laws.  Without the Grantee's prior written consent, no other securities may be
included in any such registration.  The Company shall use all reasonable efforts
to cause each such registration statement to become effective, to obtain all
consents or waivers of other parties that are required therefor and to keep such
registration effective for such period not in excess of 180 days from the day
such registration statement first becomes effective as may be as reasonably
necessary to effect such sale or other disposition.  The obligations of the
Company hereunder to file a registration statement and to maintain its
effectiveness may be suspended for one or more periods of time not exceeding 60
days in the aggregate if the Board of Directors of the Company shall have
determined in good faith that the filing of such registration or the maintenance
of its effectiveness would require disclosure of


                                         -6-


nonpublic information that would materially and adversely affect the Company. 
For purposes of determining whether two requests have been made under this
Section 8, only requests relating to a registration statement that has become
effective under the Securities Act and pursuant to which the Grantee has
disposed of all shares covered thereby in the manner contemplated therein shall
be counted.

          (b)  The expenses associated with the preparation and filing of any
such registration statement pursuant to this Section 8 and any sale covered
thereby (including any fees related to blue sky qualifications and filing fees
in respect of the National Association of Securities Dealers, Inc.)
('Registration Expenses') shall be for the account of the Company except for
underwriting discounts or commissions or brokers' fees in respect to shares to
be sold by the Grantee and the fees and disbursement of the Grantee's counsel;
PROVIDED, HOWEVER, that the Company shall not be required to pay for any
Registration Expenses with respect to such registration if the registration
request is subsequently withdrawn at the request of the Grantee unless the
Grantee agrees to forfeit its right to request one registration; AND PROVIDED
FURTHER that, if at the time of such withdrawal the Grantee has learned of a
material adverse change in the results of operations, condition (financial or
other), business or prospects of the Company from that known to the Grantee at
the time of its request and has withdrawn the request with reasonable promptness
following disclosure by the Company of such material adverse change, then the
Grantee shall not be required to pay any of such expenses and shall retain all
remaining rights to request registration.

          (c)  The Grantee shall provide all information reasonably requested by
the Company for inclusion in any registration statement to be filed hereunder. 
If during the Registration Period the Company shall propose to register under
the Securities Act the offering, sale and delivery of Company Common Stock for
cash for its own account or for any other the Company of the Company pursuant to
a firm underwriting, it shall, in addition to the Company's other obligations
under this Section 8, allow the Grantee the right to participate in such
registration provided that the Grantee participates in the underwriting;
PROVIDED, HOWEVER, that, if the managing underwriter of such offering advises
the Company in writing that in its opinion the number of shares of Company
Common Stock requested to be included in such registration exceeds the number
that can be sold in such offering, the Company shall, after fully including
therein all securities to be sold by the Company, include the shares requested
to be included therein by Grantee pro rata (based on the number of shares
intended to be included therein) with the shares intended to be included therein
by Persons other than the Company.  In connection with any offering, sale and
delivery of Company Common Stock pursuant to a registration statement effected
pursuant to this Section 8, the Company and the Grantee shall provide each


                                         -7-


other and each underwriter of the offering with customary representations,
warranties and covenants, including covenants of indemnification and
contribution.

     9.   FIRST REFUSAL.  At any time after the first occurrence of an Exercise
Event and prior to the second anniversary of the first purchase of shares of
Company Common Stock pursuant to the Option, if the Grantee shall desire to
sell, assign, transfer or otherwise dispose of all or any of the Option Shares
or other securities acquired by it pursuant to the Option, it shall give the
Company written notice of the proposed transaction (an 'Offeror's Notice'),
identifying the proposed transferee, accompanied by a copy of a binding offer to
purchase such shares or other securities signed by such transferee and setting
forth the terms of the proposed transaction.  An Offeror's Notice shall be
deemed an offer by the Grantee to the Company, which may be accepted, in whole
but not in part, within ten business days of the receipt of such Offeror's
Notice, on the same terms and conditions and at the same price at which the
Grantee is proposing to transfer such shares or other securities to such
transferee.  The purchase of any such shares or other securities by the Company
shall be settled within ten business days of the date of the acceptance of the
offer and the purchase price shall be paid to the Grantee in immediately
available funds.  If the Company shall fail or refuse to purchase all the shares
or other securities covered by an Offeror's Notice, the Grantee may, within
sixty days from the date of the Offeror's Notice, sell all but not less them
all, of such shares or other securities to the proposed transferee at no less
than the price specified and on terms no more favorable than those set forth in
the Offeror's Notice; PROVIDED, HOWEVER, that the provisions of this sentence
shall not limit the rights the Grantee may otherwise have if the Company has
accepted the offer contained in the Offeror's Notice and wrongfully refuses to
purchase the shares or other securities subject thereto.  The requirements of
this Section 9 shall not apply to (a) any disposition as a result of which the
proposed transferee would own beneficially not more than 4.9% of the outstanding
voting power of the Company, (b) any disposition of Company Common Stock or
other securities by a Person to whom the Grantee has assigned its rights under
the Option with the consent of the Company, (c) any sale by means of a public
offering registered under the Securities Act or (d) any transfer to a wholly
owned subsidiary of the Grantee which agrees in writing to be bound by the terms
hereof.

     10.  PROFIT LIMITATION.  Notwithstanding any provision to the contrary
contained in this Option Agreement, the Grantee may not exercise its rights
pursuant to this Option Agreement in a manner that would result in a cash
payment to the Grantee of an aggregate amount under this Option Agreement and
under Section 7.1(c) of the Merger Agreement of more than the sum of (a) the
aggregate exercise price paid by the Grantee for any Option Shares as to which
the Option has theretofore been exercised, PLUS (b) $18,000,000, it being
understood and agreed that to the


                                         -8-


extent that any amount is paid by the Company to the Grantee pursuant to this
Option Agreement, the fee payable pursuant to Section 7(c) of the Merger
Agreement shall be reduced appropriately so that the aggregate amount payable by
the Company under this Option Agreement and Section 7(c) of the Merger Agreement
shall not exceed such sum.

     11.  LISTING.  If at the time of the occurrence of an Exercise Event the
Company Common Stock is (or any other securities subject to the Option are) then
listed on The Nasdaq National Market or on any other market or exchange, the
Company, upon the occurrence of an Exercise Event, shall promptly file an
application to list on The Nasdaq National Market and on such other market or
exchange the shares of the Company Common Stock or other securities then subject
to the Option, and shall use all reasonable efforts to cause such listing
application to be approved as promptly as practicable.

     12.  REPLACEMENT OF AGREEMENT.  Upon receipt by the Company of evidence
reasonably satisfactory to it of the loss, theft, destruction or mutilation of
this Option Agreement, and (in the case of loss, theft or destruction) of
reasonably satisfactory indemnification, and upon surrender and cancellation of
this Option Agreement, if mutilated, the Company will execute and deliver a new
Option Agreement of like tenor and date.  

     13.  MISCELLANEOUS.

          (a)  EXTENSION; WAIVER.  At any time prior to the Termination Date,
the parties hereto may (i) extend the time for the performance of any of the
obligations or other acts of the other parties hereto, or (ii) waive compliance
with any of the agreements or conditions contained herein.  Any agreement on the
part of any party to any such extension or waiver shall be valid only if set
forth in an instrument in writing signed on behalf of such party.  Except as
provided in this Option Agreement, no action taken pursuant to this Option
Agreement shall be deemed to constitute a waiver by the party taking such action
of compliance with any covenants or agreements contained in this Option
Agreement.  The waiver by any party hereto of a breach of any provision
hereunder shall not operate or be construed as a waiver of any prior or
subsequent breach of the same or any other provision hereunder.

          (b)  NOTICES.  All notices, requests, demands, waivers and other
communications required or permitted to be given under this Option Agreement
shall be in writing and shall be deemed to have been duly given if delivered in
person or mailed, certified or registered mail with postage prepaid, or sent by
telex, telegram or telecopier, as follows:


                                         -9-


                         if to the Company, to it at:

                         NeXstar Pharmaceuticals, Inc.
                         2860 Wilderness Place
                         Boulder, Colorado 80301
                         Attention: Chief Financial Officer
                         Telecopy No.: (303) 546-7603

                         with a copy to:

                         Willkie Farr & Gallagher
                         787 Seventh Avenue
                         New York, New York 10019
                         Attention: Peter H. Jakes, Esq.
                         Telecopy No.: (212) 728-8111

                         if to either Parent or Sub, to it at:

                         Gilead Sciences, Inc
                         333 Lakeside Drive
                         Foster City, CA 94404
                         Attn:  General Counsel

                         Telecopy No. : (650) 522-5622

                         with a copy to:

                         Cooley Godward LLP
                         Five Palo Alto Square
                         3000 E1 Camino Real
                         Palo Alto, CA 94306-2155
                         Attention:  Richard E. Climan, Esq.
                                     Keith A. Flaum, Esq.
                         Telecopy No.:  (650) 857-0663

or to such other Person or address as any party shall specify by notice in
writing to each of the other parties.  All such notices, requests, demands,
waivers and communications shall be deemed to have been received on the date of
delivery unless if mailed, in which case on the third business day (fifth
business day, if mailed outside the country of the recipient) after the mailing
thereof except for a notice of a change of address, which shall be effective
only upon receipt thereof.

          (c)  ENTIRE AGREEMENT.  This Option Agreement and the other documents
referred to herein or delivered pursuant hereto collectively contain the entire
understanding of the parties hereto with respect to the subject matter contained
herein and therein and supersede all prior agreements and understandings, oral
and written, with respect thereto.

          (d)  BINDING EFFECT; BENEFIT; ASSIGNMENT.  This Option Agreement shall
inure to the benefit of and be binding upon the parties hereto and their
respective successors and permitted


                                         -10-


assigns, but neither this Option Agreement nor any of the rights, interest or
obligations hereunder shall be assigned by any of the parties hereto without the
prior written consent of the other parties.  Notwithstanding anything contained
in this Option Agreement to the contrary, nothing in this Option Agreement,
expressed or implied, is intended to confer on any Person other than the parties
hereto or their respective successors and assigns any rights, remedies,
obligations or liabilities under or by reason of this Option Agreement.

          (e)  AMENDMENT AND MODIFICATION.  Subject to applicable law, this
Option Agreement may be amended, modified and supplemented, or provisions hereof
waived, in writing by the parties hereto in any and all respects before the
Termination Date, by action taken by the respective Boards of Directors of the
Company or the Grantee or by the respective officers authorized by such Boards
of Directors.  This Option Agreement may not be amended except by an instrument
in writing signed on behalf of each of the parties hereto.

          (f)  FURTHER ACTIONS.  Each of the parties hereto agrees that, subject
to its legal obligations, it will use its reasonable efforts to do all things
reasonably necessary to consummate the transactions contemplated hereby.

          (g)  HEADINGS.  The descriptive headings of the several Sections of
this Option Agreement are inserted for convenience only, do not constitute a
part of this Option Agreement and shall not affect in any way the meaning or
interpretation of this Option Agreement.

          (h)  COUNTERPARTS.  This Option Agreement may be executed in several
counterparts, each of which shall be deemed to be an original, and all of which
together shall be deemed to be one and the same instrument.

          (i)  APPLICABLE LAW.  This Option Agreement and the legal relations
between the parties hereto shall be governed by and construed in accordance with
the laws of the State of Delaware, without regard to the conflict of laws rules
thereof.

          (j)  SEVERABILITY.  If any term, provision, covenant or restriction
contained in this Option Agreement is held by a court of competent jurisdiction
or other authority to be invalid, void, unenforceable or against its regulatory
policy, the remainder of the terms, provisions, covenants and restrictions
contained in this Option Agreement shall remain in full force and effect and
shall in no way be affected, impaired or invalidated.

          (k)  ENFORCEMENT OF AGREEMENT.  The parties hereto agree that
irreparable damage would occur in the event that any of the provisions of this
Option Agreement was not performed in accordance with its specific terms or was
otherwise breached.  It is accordingly agreed that the parties shall be entitled
to an


                                         -11-
 :  PAGE>

injunction or injunctions to prevent breaches of this Option Agreement and to
enforce specifically the terms and provisions hereof in any Delaware Court, this
being in addition to any other remedy to which they are entitled at law or in
equity.

          (l)  SUBMISSION TO JURISDICTION.  With respect to any suit, action or
proceeding initiated by a party to this Option Agreement arising out of, under
or in connection with this Option Agreement, the Company and the Grantee each
hereby submit to the non-exclusive jurisdiction of any state or federal court
sitting in the State of Delaware and irrevocably waive, to the fullest extent
permitted by law, any objection that they may now have or hereafter obtain to
the laying of venue in any such court in any such suit, action or proceeding.

          (m)  ATTORNEYS' FEES. If any suit, action or proceeding relating to
this Option Agreement or the enforcement of any provision of this Option
Agreement is brought against the Company, the prevailing party shall be entitled
to recover reasonable attorneys' fees, costs and disbursements (in addition to
any other relief to which the prevailing party may be entitled).

          (n)  NON-EXCLUSIVITY.  The rights and remedies of the Grantee under
this Option Agreement are not exclusive of or limited by any other rights or
remedies which it may have, whether at law, in equity, by contract or otherwise,
all of which shall be cumulative (and not alternative).  Without limiting the
generality of the foregoing, the rights and remedies of the Grantee under this
Option Agreement, and the obligations and liabilities of the Company under this
Option Agreement, are in addition to their respective rights, remedies,
obligations and liabilities under common law requirements and under all
applicable statutes, rules and regulations. The covenants and obligations of the
Company set forth in this Option Agreement shall be construed as independent of
any other agreement or arrangement between the Company, on the one hand, and the
Grantee, on the other.  The existence of any claim or cause of action by the
Company against the Grantee shall not constitute a defense to the enforcement of
any of such covenants or obligations against the Company.




                                         -12-


          IN WITNESS WHEREOF, the Company and the Grantee have caused this
Option Agreement to be signed by their respective officers thereupon duly
authorized, all as of the day and year first written above.



                              GILEAD SCIENCES, INC.:

                              By: /s/ Mark L. Perry
                                 -----------------------------
                              Name: Mark L. Perry
                                   ---------------------------
                              Title: Senior Vice President and
                                     Chief Financial Officer
                                    --------------------------

                              NEXSTAR PHARMACEUTICALS, INC.:

                              By: /s/ Michael E. Hart
                                 -----------------------------
                              Name: Michael E. Hart
                                   ---------------------------
                              Title: Vice President and
                                     Chief Financial Officer
                                    --------------------------


                                         -13-