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                                                                     EXHIBIT 4.1
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                            STOCK PURCHASE AGREEMENT

         This STOCK PURCHASE AGREEMENT (the "Agreement") is made as of January
8, 2001, by and between OSI PHARMACEUTICALS, INC., a Delaware corporation (the
"Company"), and GENENTECH, INC., a Delaware corporation (the "Investor").

                                    RECITALS

         A. The Company, the Investor, and F.Hoffmann-La Roche Ltd have this day
entered into various development and marketing collaboration and license
agreements (collectively, the "Collaboration Agreements") covering the proposed
development and commercialization of the Company's anti-cancer drug OSI-774.

         B. In connection with the Collaboration Agreements, the Company, the
Investor and Roche Holdings, Inc. ("Roche") have agreed that the Investor and
Roche will each purchase 462,570 shares of the Company's common stock, $.01 par
value, for an aggregate purchase price of $35,000,000 each.

         C. The Company and the Investor are executing and delivering this
Agreement in reliance upon the exemption from registration under the Securities
Act of 1933, as amended, by virtue of Section 4(2) thereof and Regulation D, as
promulgated thereunder by the U.S. Securities and Exchange Commission.

         NOW, THEREFORE, in consideration of the premises and the mutual
promises made herein and for other good and valuable consideration, the receipt
and sufficiency of which is hereby acknowledged, the parties hereto, intending
to be legally bound, agree as follows:

         1. Definitions. In addition to those terms defined above and elsewhere
in this Agreement, for the purposes of this Agreement, the following terms shall
have the meanings set forth here:

                  1.1. "Affiliate" shall mean, with respect to any Person, any
other Person which directly or indirectly controls, is controlled by, or is
under common control with, such Person. Notwithstanding the foregoing, for the
purposes of this Agreement, F.Hoffmann-La Roche Ltd and all entities presently
or in the future controlled by, controlling, or under common control with
F.Hoffmann-La Roche Ltd, except for the Investor or any entity controlled by the
Investor, shall not be considered an Affiliate of the Investor. For purposes of
this definition, the "control" of a company shall mean either the ownership of a
majority of the outstanding equity capital of the company or the ownership of
(or right to acquire within 60 days) equity capital of the company which would
entitle the owner to elect a majority of the board of directors or other
governing body of such company.

                  1.2. "Closing" shall have the meaning set forth in Section 3.

                  1.3. "Closing Date" shall have the meaning set forth in
Section 3.

                  1.4. "Common Stock" shall mean the common stock of the
Company, $0.01 par value per share.

                  1.5. "Environmental Laws" shall have the meaning set forth in
Section 4.18.
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                  1.6. "Exchange Act" shall mean the Securities Exchange Act of
1934, as amended, and the rules and regulations promulgated thereunder.

                  1.7. "FTC" shall mean the U.S. Federal Trade Commission.

                  1.8. "Hart-Scott-Rodino Act" shall mean the Hart-Scott-Rodino
Antitrust Improvements Act of 1976, as amended, and the rules and regulations
promulgated thereunder.

                  1.9. "Intellectual Property" shall have the meaning set forth
in Section 4.8.

                  1.10. "Material Adverse Effect" shall mean a material adverse
effect on the (i) condition (financial or otherwise), business, assets, or
results of operations of the Company and its subsidiaries, taken as a whole;
(ii) ability of the Company to perform any of its material obligations under the
terms of this Agreement; or (iii) rights and remedies of the Investor under the
terms of this Agreement.

                  1.11. "NASD" shall mean the National Association of Securities
Dealers, Inc.

                  1.12. "Nasdaq National Market" shall mean The Nasdaq Stock
Market, Inc. National Market.

                  1.13. "Person" shall mean an individual, corporation,
partnership, trust, business trust, association, joint stock company, joint
venture, pool, syndicate, sole proprietorship, unincorporated organization,
governmental authority or any other form of entity not specifically listed
herein.

                  1.14. "Purchase Price" shall have the meaning set forth in
Section 2.

                  1.15. "Registration Statement" shall have the meaning set
forth in Section 9.1(a).

                  1.16. "Roche Stock Purchase Agreement" shall have the meaning
set forth in Section 7.1(e).

                  1.17. "SEC" shall mean the U.S. Securities and Exchange
Commission.

                  1.18. "SEC Documents" shall have the meaning set forth in
Section 4.12.

                  1.19. "Securities Act" shall mean the Securities Act of 1933,
as amended, and the rules and regulations promulgated thereunder.

                  1.20. "Shares" shall have the meaning set forth in Section 2.

                  1.21. "Suspension" shall have the meaning set forth in Section
9.2(c).

                  1.22. "Suspension Notice" shall have the meaning set forth in
Section 9.2(c).

                  1.23. "Voting Securities" shall mean all classes of capital
stock of the Company which are then entitled to vote generally in the election
of directors.


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         2. Purchase and Sale of the Shares. Subject to the terms and conditions
of this Agreement, the Investor hereby agrees to purchase from the Company and
the Company hereby agrees to sell and issue to the Investor 462,570 shares of
Common Stock (the "Shares") for an aggregate purchase price (the "Purchase
Price") of $35,000,000 (approximately $75.664 per share).

         3. Delivery of the Shares at Closing. The closing of the purchase and
sale of the Shares (the "Closing") shall occur on the earlier of (a) the fifth
day, unless the fifth day falls on a weekend or holiday, in which case it shall
be the next business day, after the earlier of (i) receipt of written notice by
the Investor, Roche and the Company of the FTC's approval of the transactions
contemplated by this Agreement and the Collaboration Agreements pursuant to the
filings made by the Company, F.Hoffmann-La Roche Ltd and the Investor under the
Hart-Scott-Rodino Act and (ii) the expiration or termination of all applicable
waiting periods, requests for information (and any extensions thereof) under the
Hart-Scott-Rodino Act for such filings, and (b) the fifth day, unless the fifth
day falls on a weekend or holiday, in which case it shall be the next business
day, after the joint determination (by written opinion from each party to the
other) that filings under the Hart-Scott-Rodino Act are not required (the
"Closing Date"). The Closing shall occur at the offices of Saul Ewing LLP,
Centre Square West, 1500 Market Street, 38th Floor, Philadelphia, Pennsylvania,
or at such other place as the parties shall mutually agree upon. At the Closing,
the Company shall deliver to the Investor one or more stock certificates
representing the Shares, each such certificate to be registered in the name of
the Investor or, if so indicated on the signature page hereto, in the name of a
nominee designated by the Investor, and the Investor shall wire the Purchase
Price in immediately available funds to an account designated by the Company.

         4. Representations and Warranties of the Company. The Company hereby
represents and warrants to the Investor as follows:

                  4.1. Organization. The Company and each of its subsidiaries is
duly organized and validly existing in good standing under the laws of the
jurisdiction of its organization. Each of the Company and its subsidiaries has
full power and authority to own, operate and occupy its properties and to
conduct its business as presently conducted. Each of the Company and its
subsidiaries is registered or qualified to do business and is in good standing
in each jurisdiction in which it owns or leases property or transacts business
and where the failure to be so qualified would have a Material Adverse Effect.
No proceeding has been instituted in any such jurisdiction, revoking, limiting
or curtailing, or seeking to revoke, limit or curtail, such power and authority
or qualification.

                  4.2. Due Authorization and Valid Issuance. The Company has all
requisite power and authority to execute, deliver and perform its obligations
under this Agreement. The Agreement has been duly authorized and validly
executed and delivered by the Company and constitutes a legal, valid and binding
agreement of the Company enforceable against the Company in accordance with its
terms, except as (a) rights to indemnity and contribution may be limited by
state or federal securities laws or the public policy underlying such laws, (b)
enforceability may be limited by applicable bankruptcy, insolvency,
reorganization, moratorium or similar laws affecting creditors' and contracting
parties' rights generally, and (c) enforceability may be subject to general
principles of equity (regardless of whether such enforceability is considered in
a proceeding in equity or at law). The Shares have been duly authorized, and
will, upon issuance pursuant to the terms hereof, be duly and validly issued,
fully paid and nonassessable and free and clear of all encumbrances and
restrictions on transfer, except for restrictions on transfer imposed by this
Agreement or by applicable federal or state securities laws.


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                  4.3. Non-Contravention. The execution and delivery of this
Agreement, the issuance and sale of the Shares to be sold by the Company under
this Agreement, the fulfillment of the terms of this Agreement and the
consummation of the transactions contemplated hereby will not (a) conflict with
or constitute a violation of, or default (with the passage of time or otherwise)
under, (i) any material bond, debenture, note or other evidence of indebtedness,
lease, contract, indenture, mortgage, deed of trust, loan agreement, joint
venture or other agreement or instrument to which the Company or any of its
subsidiaries is a party or by which it or any of its subsidiaries or their
respective properties are bound, (ii) the charter, by-laws or other
organizational documents of the Company or any subsidiary, or (iii) any law,
administrative regulation, ordinance or order of any court or governmental
agency, arbitration panel or authority applicable to the Company or any
subsidiary or their respective properties, or (b) result in the creation or
imposition of any lien, encumbrance, claim, security interest or restriction
whatsoever upon any of the material properties or assets of the Company or any
subsidiary or an acceleration of indebtedness pursuant to any obligation,
agreement or condition contained in any material bond, debenture, note or any
other evidence of indebtedness or any material indenture, mortgage, deed of
trust or any other agreement or instrument to which the Company or any
subsidiary is a party or by which any of them is bound or to which any of the
property or assets of the Company or any subsidiary is subject. No consent,
approval, authorization or other order of, or registration, qualification or
filing with, any regulatory body, administrative agency, or other governmental
body in the United States or any other Person is required for the execution and
delivery of this Agreement and the valid issuance and sale of the Shares, other
than such as have been made or obtained, and except for any securities filings
or notifications required to be made after the Closing under federal or state
securities laws.

                  4.4. Capitalization. The authorized capital of the Company
consists of 50,000,000 shares of Common Stock, of which approximately 34,561,196
shares are issued and outstanding, and 5,000,000 shares of Preferred Stock, $.01
par value, none of which are issued and outstanding. The financial statements of
the Company and the related notes contained in the SEC Documents set forth the
number of shares of capital stock issuable pursuant to the Company's stock
option and stock purchase plans and the number of shares of capital stock
issuable and reserved for issuance pursuant to the Company's securities
exercisable for, or convertible into or exchangeable for any shares of capital
stock. The outstanding shares of capital stock of the Company have been duly
authorized and validly issued and are fully paid and nonassessable, have been
issued in compliance with all federal and state securities laws, and were not
issued in violation of any preemptive rights or similar rights to subscribe for
or purchase securities. With the exception of options and rights to acquire
stock pursuant to the Company's stock option and stock purchase plans, certain
warrants held by employees of the Company's United Kingdom subsidiary, and
rights under the Company's Shareholder Rights Plan, there are no outstanding
rights (including, without limitation, preemptive rights), warrants or options
to acquire, or instruments convertible into or exchangeable for, any unissued
shares of capital stock or other equity interest in the Company or any
subsidiary, or any contract, commitment, agreement, understanding or arrangement
of any kind to which the Company is a party or of which the Company has
knowledge and relating to the issuance or sale of any capital stock of the
Company or any subsidiary, any such convertible or exchangeable securities or
any such rights, warrants or options. Without limiting the foregoing, except for
certain registration rights held by Novartis Pharma AG and the rights granted to
Roche in the Roche Stock Purchase Agreement, no preemptive right, co-sale right,
right of first refusal, registration right, or other similar right exists with
respect to the Shares or the issuance and sale thereof. No further approval or
authorization of any stockholder of the Company, the Board of Directors of the
Company or others is required for the issuance and sale of the Shares. The
Company owns the entire equity interest in each of its subsidiaries, free and
clear of any pledge, lien, security interest, encumbrance, claim or equitable
interest. There are no stockholders agreements, voting agreements or

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other similar agreements with respect to the Common Stock to which the Company
is a party or, to the knowledge of the Company, between or among any of the
Company's stockholders.

                  4.5. Legal Proceedings. There is no material legal or
governmental proceeding pending or, to the knowledge of the Company, threatened
to which the Company or any subsidiary is or may be a party or of which the
business or property of the Company or any subsidiary is subject.

                  4.6. No Violations. Neither the Company nor any subsidiary is
in violation of its charter, by-laws or other organizational document, or in
violation of any law, administrative regulation, ordinance or order of any court
or governmental agency, arbitration panel or authority applicable to the Company
or any subsidiary, which violation, individually or in the aggregate, would be
reasonably likely to have a Material Adverse Effect or is in default (and there
exists no condition which, with the passage of time or otherwise, would
constitute a default) in any material respect in the performance of any bond,
debenture, note or any other evidence of indebtedness in any indenture,
mortgage, deed of trust or any other material agreement or instrument to which
the Company or any subsidiary is a party or by which the Company or any
subsidiary is bound or by which the properties of the Company or any subsidiary
are bound, which would be reasonably likely to have a Material Adverse Effect.

                  4.7. Governmental Permits, Etc. Each of the Company and its
subsidiaries has all necessary franchises, licenses, certificates and other
authorizations from any foreign, federal, state or local government or
governmental agency, department, or body that are currently necessary for the
operation of the business of the Company and its subsidiaries as currently
conducted except where the failure to currently possess could not reasonably be
expected to have a Material Adverse Effect.

                  4.8. Intellectual Property. The Company and its subsidiaries
own or possess sufficient rights to use all material patents, patent rights,
trademarks, copyrights, licenses, inventions, trade secrets, trade names and
know-how (collectively, "Intellectual Property") as owned or possessed by it or
that are necessary for the conduct of its business as now conducted or as
proposed to be conducted except where the failure to currently own or possess
would not have a Material Adverse Effect. The Company has taken all commercially
reasonable measures to protect its ownership or possession of the Intellectual
Property. Neither the Company nor any of its subsidiaries has received any
notice of, or has any knowledge of, any infringement of asserted rights of a
third party with respect to any Intellectual Property that, individually or in
the aggregate, would have a Material Adverse Effect, and neither the Company nor
any of its subsidiaries has received any notice of any infringement of rights of
a third party with respect to any Intellectual Property that, individually or in
the aggregate, would have a Material Adverse Effect.

                  4.9. Financial Statements. The financial statements of the
Company and the related notes contained in the SEC Documents present fairly, in
accordance with generally accepted accounting principles, the financial position
of the Company and its subsidiaries as of the dates indicated, and the results
of its operations and cash flows for the periods therein specified. Such
financial statements (including the related notes) have been prepared in
accordance with generally accepted accounting principles. To the Company's
knowledge, there are no material liabilities that are not set forth in the
financial statements of the Company and the related notes contained in the SEC
Documents, other than liabilities which have incurred in the regular course of
business of the Company.

                  4.10. No Material Adverse Change. Since the filing of the
Company's most recent Annual Report on Form 10-K, there has not been (a) any
material adverse change in the financial condition or earnings of the Company
and its subsidiaries considered as one enterprise nor has any material

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adverse event occurred to the Company or its subsidiaries, (b) any material
adverse event affecting the Company, (c) any obligation, direct or contingent,
that is material to the Company and its subsidiaries considered as one
enterprise, incurred by the Company, except obligations incurred in the ordinary
course of business, (d) any dividend or distribution of any kind declared, paid
or made on the capital stock of the Company or any of its subsidiaries, or (e)
any loss or damage (whether or not insured) to the physical property of the
Company or any of its subsidiaries which has been sustained, which has a
Material Adverse Effect.

                  4.11. Nasdaq Compliance. The Company's Common Stock is
registered pursuant to Section 12(g) of the Exchange Act and is listed on the
Nasdaq National Market, and the Company has taken no action designed to, or
likely to have the effect of, terminating the registration of the Common Stock
under the Exchange Act or de-listing the Common Stock from the Nasdaq National
Market, nor has the Company received any notification that the SEC or the NASD
is contemplating terminating such registration or listing.

                  4.12. Reporting Status. The Company has filed in a timely
manner all documents that the Company was required to file under the Exchange
Act during the twelve months preceding the date of this Agreement. The following
documents (the "SEC Documents") complied in all material respects with the SEC's
requirements as of their respective filing dates, and the information contained
therein as of the date thereof did not contain an untrue statement of a material
fact, or omit to state a material fact required to be stated therein, or
necessary to make the statements therein in light of the circumstances under
which they were made, not misleading:

                         (a) The Company's Annual Report on Form 10-K for the
year ended September 30, 2000, filed with the SEC on December 19, 2000;

                         (b) The Company's Definitive Proxy Statement filed with
the SEC on January 28, 2000, in connection with the 2000 Annual Meeting of
Stockholders;

                         (c) The Company's Current Report on Form 8-K filed with
the SEC on November 9, 2000; and

                         (d) All other documents, if any, filed by the Company
with the SEC since September 30, 2000, pursuant to the reporting requirements of
the Exchange Act.

                  4.13. Contracts. The contracts described in the SEC Documents
or incorporated by reference therein that are material to the Company are in
full force and effect on the date hereof, and neither the Company nor, to the
Company's knowledge, any other party to such contracts is in breach of or
default under any of such contracts which breach or default would have a
Material Adverse Effect.

                  4.14. Taxes. The Company has filed all necessary federal,
state and foreign income and franchise tax returns and has paid or accrued all
taxes shown as due thereon, and the Company has no knowledge of a tax deficiency
which has been or might be asserted or threatened against it which would have a
Material Adverse Effect.

                  4.15. Investment Company. The Company is not an "investment
company" or an "affiliated person" of, or "promoter" or "principal underwriter"
for an investment company, within the meaning of the Investment Company Act of
1940, as amended.


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                  4.16. Title to Property. The Company and its subsidiaries have
good and marketable title to all real properties and all other properties and
assets owned by them, in each case free from liens, encumbrances and defects
that would materially affect the value thereof or materially interfere with the
use made or currently planned to be made thereof by them; and the Company and
its subsidiaries hold any leased, real or personal property under valid and
enforceable leases with no exceptions that would materially interfere with the
use made or currently planned to be made thereof by them.

                  4.17. No Labor Disputes. No material labor dispute with the
employees of the Company or any subsidiary exists or, to the knowledge of the
Company, is imminent.

                  4.18. Environmental Matters. Neither the Company nor any of
its subsidiaries is in violation of any statute, rule, regulation, decision or
order of any governmental agency or body or any court, domestic or foreign,
relating to the use, disposal or release of hazardous or toxic substances or
relating to the protection or restoration of the environment or human exposure
to hazardous or toxic substances (collectively, "Environmental Laws"), owns or
operates any real property contaminated with any substance that is subject to
any Environmental Laws, is liable for any off-site disposal or contamination
pursuant to any Environmental Laws, or is subject to any claim relating to any
Environmental Laws, which violation, contamination, liability or claim would
individually or in the aggregate have a Material Adverse Effect; and the Company
is not aware of any pending investigation that might lead to such a claim.

                  4.19. Insurance. The Company maintains insurance of the types
and in the amounts that the Company reasonably believes is adequate for its
business, including, but without limitation, product liability insurance,
insurance covering all real and personal property owned or leased by the Company
against theft, damage, destruction and all other risks customarily insured
against by similarly situated companies, all of which insurance is in full force
and effect.

                  4.20. Disclosure. No representation or warranty made under any
Section hereof contains an untrue statement of a material fact or omits to state
a material fact necessary to make the statements herein, in light of the
circumstances under which the statements were made, not misleading.

         5. Representations, Warranties and Covenants of the Investor.

                  5.1. Representations and Warranties. The Investor hereby
represents and warrants to the Company as follows:

                         (a) Organization and Existence. The Investor is a
corporation duly organized and validly existing in good standing under the laws
of the jurisdiction of its organization and has all requisite corporate power
and authority to purchase the Shares pursuant to this Agreement.

                         (b) Accredited Investor, Investment Decision.

                              (i) The Investor is an "accredited investor" as
defined in Regulation D under the Securities Act and is knowledgeable,
sophisticated and experienced in making investments of the type contemplated by
this Agreement.

                              (ii) The Investor has requested, received,
reviewed and considered all information it has deemed relevant in making an
informed decision to purchase the Shares.


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                              (iii) The Investor is acquiring the Shares in the
ordinary course of its business and for its own account for investment only and
with no present intention of distributing any of the Shares and has no
arrangement or understanding with any other Persons regarding the distribution
of the Shares.

                              (iv) In connection with its decision to purchase
the Shares, the Investor has relied only upon the SEC Documents and the
representations and warranties of the Company contained herein.

                              (v) The Investor understands that its acquisition
of the Shares has not been registered under the Securities Act or registered or
qualified under any state securities law in reliance on specific exemptions
therefrom, which exemptions may depend upon, among other things, the bona fide
nature of the Investor's investment intent as expressed in this Agreement.

                         (c) Due Authorization.

                              (i) The Investor has full right, power, authority
and capacity to enter into this Agreement and to consummate the transactions
contemplated hereby and has taken all necessary action to authorize the
execution, delivery and performance of this Agreement.

                              (ii) This Agreement constitutes a valid and
binding obligation of the Investor enforceable against the Investor in
accordance with its terms, except that (1) enforceability may be limited by
applicable bankruptcy, insolvency, reorganization, moratorium or similar laws
affecting creditors' and contracting parties' rights generally, (2)
enforceability may be subject to general principles of equity (regardless of
whether such enforceability is considered in a proceeding in equity or at law),
and (iii) the indemnification agreements of the Investor herein may be legally
unenforceable.

                  (d) NASD. The Investor has no direct or indirect affiliation
or association with any member of the NASD as of the date hereof.

                  (e) Restricted Securities, Legends. The Investor understands
that the Shares are characterized as "restricted securities" under the
Securities Act inasmuch as they are being acquired from the Company in a
transaction not involving a public offering and that under such laws and
applicable regulations such securities may be resold without registration under
the Securities Act only in certain limited circumstances. The Investor
understands that certificates evidencing the Shares may bear one or all of the
following legends:

                  "The shares represented by this certificate have not been
                  registered under the Securities Act of 1933, as amended (the
                  "Act"), and may not be transferred without (i) an opinion of
                  counsel satisfactory to the corporation that such transfer may
                  lawfully be made without registration under such Act or
                  qualification under applicable state securities laws; or (ii)
                  such registration or qualification, except for a transfer in
                  compliance with Rule 144 under the Act."

                  If required by the authorities of any state in connection with
                  the issuance of sale of the Shares, the legend required by
                  such state authority.


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Any purchaser of the Shares pursuant to an effective registration statement
under the Securities Act will be entitled to receive a certificate bearing no
restrictive legend.

                  5.2. Covenants. The Investor hereby covenants with the Company
as follows:

                         (a) The Investor will not, directly or indirectly,
offer, sell, pledge, transfer or otherwise dispose of (or solicit any offers to
buy, purchase or otherwise acquire or take a pledge of) any of the Shares except
in compliance with the Securities Act and applicable state securities laws. In
furtherance thereof, the Investor will not make any disposition of the Shares
except (x) pursuant to a registration statement under the Securities Act
covering such proposed disposition, (y) upon prior notice to the Company and, if
reasonably requested by the Company, delivery to the Company of an opinion of
counsel, reasonably satisfactory to the Company, that such disposition will not
require registration under the Securities Act, or (z) in compliance with Rule
144 under the Securities Act.

                         (b) The Investor acknowledges and agrees that no action
has been or will be taken in any jurisdiction outside the United States by the
Company that would permit an offering of the Shares, or possession or
distribution of offering materials in connection with the issuance of the
Shares, in any jurisdiction outside the United States where legal action by the
Company for that purpose is required. The Investor outside the United States
will comply with all applicable laws and regulations in each foreign
jurisdiction in which it purchases, offers, sells or delivers Shares or has in
its possession or distributes any offering material, in all cases at its own
expense.

         6. Covenants Pending Closing.

                  6.1. Cooperation.


                         (a) Reasonable Efforts. Subject to the terms and
conditions of this Agreement, each of the parties hereto agrees to use all
reasonable efforts to take, or cause to be taken, all reasonable actions and to
do, or cause to be done, all things necessary and appropriate to satisfy all
conditions of and to consummate the transactions contemplated by this Agreement.

                         (b) Filings. The parties shall cooperate with one
another in the preparation, execution and filing of all documents that are
required or permitted to be filed on or before the Closing, including without
limitation, filings pursuant to the Hart-Scott-Rodino Act.

                  6.2. Listing. The Company shall comply with all requirements
of the NASD with respect to the issuance of the Shares and the listing thereof
on the Nasdaq National Market.

         7. Conditions to Closing.

                  7.1. Conditions to Purchasers' Obligations at the Closing. The
Investor's obligations to purchase the Shares at the Closing are subject to the
satisfaction, at or prior to the Closing, of the following conditions:

                         (a) Representations and Warranties True; Performance of
Obligations. The representations and warranties made by the Company in Section 3
hereof shall be true and correct in all material respects as of the Closing Date
with the same force and effect as if they had been made as of the Closing Date,
and the Company shall have performed all obligations and conditions herein
required to be performed or observed by it on or prior to the Closing.


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                         (b) Consents, Permits, and Waivers. The Company shall
have obtained any and all consents, permits and waivers necessary or appropriate
for consummation of the transactions contemplated by this Agreement (except for
such as may be properly obtained subsequent to the Closing), including, without
limitation, approval by the FTC pursuant to the Hart-Scott Rodino Act.

                         (c) Compliance Certificate. The Company shall have
delivered to the Investor a Compliance Certificate, executed by the Chief
Executive Officer or the President of the Company, dated the Closing Date, to
the effect that the conditions specified in subsections (a) and (b) of this
Section 7.1 have been satisfied.

                         (d) Collaboration Agreements. The Collaboration
Agreements shall have been executed and delivered by the Company, the Investor,
and F.Hoffmann-La Roche Ltd, and such agreement shall be fully effective.

                         (e) Roche Stock Purchase Agreement. Closing shall have
occurred (or shall occur simultaneously with the Closing hereunder) under the
Stock Purchase Agreement, dated as of January 8, 2001, between the Company and
Roche (the "Roche Stock Purchase Agreement").

                         (f) Legal Opinion. The Investor shall have received
from Saul Ewing LLP, legal counsel to the Company, an opinion addressed to them,
dated as of the Closing Date, in substantially the form attached hereto as
Exhibit A.

                         (g) Proceedings and Documents. All corporate and other
proceedings in connection with the transactions contemplated at the Closing
hereby and all documents and instruments incident to such transactions shall be
reasonably satisfactory in substance and form to the Investor and its counsel,
and the Investor and its counsel shall have received all such counterpart
originals or certified or other copies of such documents as they may reasonably
request.

                  7.2. Conditions to Obligations of the Company. The Company's
obligation to issue and sell the Shares at the Closing is subject to the
satisfaction, at or prior to the Closing, of the following conditions:

                         (a) Representations and Warranties True; Performance of
Obligations. The representations and warranties made by the Investor in Section
4 hereof shall be true and correct in all material respects as of the Closing
Date with the same force and effect as if they had been made as of the Closing
Date, and the Investor shall have performed all obligations and conditions
herein required to be performed or observed by it on or prior to the Closing.

                         (b) Consents, Permits, and Waivers. The Company shall
have obtained any and all consents, permits and waivers necessary or appropriate
for consummation of the transactions contemplated by this Agreement (except for
such as may be properly obtained subsequent to the Closing), including, without
limitation, approval by the FTC pursuant to the Hart-Scott Rodino Act.

                         (c) Collaboration Agreements. The Collaboration
Agreements shall have been executed and delivered by the Company, the Investor,
and F.Hoffmann-La Roche Ltd, and such agreement shall be fully effective.

                         (d) Roche Stock Purchase Agreement. Closing shall have
occurred (or shall occur simultaneously with the Closing hereunder) under the
Roche Stock Purchase Agreement.


                                       10
<PAGE>   12
                         (e) Proceedings and Documents. All corporate and other
proceedings in connection with the transactions contemplated at the Closing
hereby and all documents and instruments incident to such transactions shall be
reasonably satisfactory in substance and form to the Company and its counsel,
and the Company and its counsel shall have received all such counterpart
originals or certified or other copies of such documents as they may reasonably
request.

         8. Survival of Representations and Warranties. Notwithstanding any
investigation made by either party to this Agreement, the representations and
warranties made by the Company as of the date hereof in Sections 4.1 (the first
sentence only), 4.2, 4.3, 4.6, 4.8, 4.14, 4.18, and 4.20 and the representations
and warranties made by the Investor shall survive the Closing indefinitely. All
other representations and warranties made by the Company herein shall survive
the Closing for a period of two years.

         9. Registration of the Shares; Compliance With the Securities Act.

                  9.1. Registration Procedures and Expenses. The Company shall:

                         (a) subject to receipt of necessary information from
the Investor after prompt request from the Company to the Investor to provide
such information, use commercially reasonable efforts to prepare and file with
the SEC, as soon as practicable after the Closing, a shelf registration
statement (the "Registration Statement") to enable the resale of the Shares by
the Investor from time to time on a delayed or continuous basis pursuant to Rule
415 of the Securities Act through the automated quotation system of the Nasdaq
National Market or such other market as may be the principal market on which the
Company's Common Stock is sold, or any other manner reasonably requested by the
Investor, including privately-negotiated transactions;

                         (b) use commercially reasonable efforts, subject to
receipt of necessary information from the Investor after prompt request from the
Company to the Investor to provide such information, to cause the Registration
Statement to become effective as soon as practicable after the Registration
Statement is filed by the Company;

                         (c) use commercially reasonable efforts to prepare and
file with the SEC such amendments and supplements to the Registration Statement
and the prospectus used in connection therewith as may be necessary to keep the
Registration Statement current and effective for a period not exceeding the
earlier of (i) the second anniversary of the Closing Date, or (ii) such time as
all Shares purchased by such Investor have been sold pursuant to the
Registration Statement;

                         (d) furnish to the Investor such number of copies of
the Registration Statement, prospectuses and preliminary prospectuses in
conformity with the requirements of the Securities Act and such other documents
as the Investor may reasonably request, in order to facilitate the public sale
or other disposition of all or any of the Shares by the Investor; provided,
however, that the obligation of the Company to deliver copies of prospectuses or
preliminary prospectuses to the Investor shall be subject to the receipt by the
Company of reasonable assurances from the Investor that the Investor will comply
with the applicable provisions of the Securities Act and of such other
securities or blue sky laws as may be applicable in connection with any use of
such prospectuses or preliminary prospectuses;

                         (e) file documents required of the Company for normal
blue sky clearance in states specified in writing by the Investor; provided,
however, that the Company shall not be

                                       11
<PAGE>   13
required to qualify to do business or consent to service of process in any
jurisdiction in which it is not now so qualified or has not so consented;

                         (f) bear all expenses in connection with the procedures
in paragraph (a) through (e) of this Section 9.1 and the registration of the
Shares pursuant to the Registration Statement; and

                         (g) advise the Investor, promptly after it shall
receive notice or obtain knowledge of the issuance of any stop order by the SEC
delaying or suspending the effectiveness of the Registration Statement or of the
initiation or threat of any proceeding for that purpose; and it will promptly
use commercially reasonable efforts to prevent the issuance of any stop order or
to obtain its withdrawal at the earliest possible moment if such stop order
should be issued.

         The Company understands that the Investor disclaims being an
underwriter, but the Investor's being deemed an underwriter by the SEC shall not
relieve the Company of any obligations it has hereunder.

                  9.2. Transfer of Shares After Registration; Suspension.

                         (a) The Investor agrees that it will not effect any
disposition of the Shares or its right to purchase the Shares that would
constitute a sale within the meaning of the Securities Act except as
contemplated in the Registration Statement referred to in Section 9.1 and as
described below or as otherwise permitted by law, and that it will promptly
notify the Company of any changes in the information set forth in the
Registration Statement regarding the Investor or its plan of distribution.

                         (b) Except in the event that paragraph (c) below
applies, the Company shall (i) if deemed necessary by the Company, prepare and
file from time to time with the SEC a post-effective amendment to the
Registration Statement or a supplement to the related prospectus or a supplement
or amendment to any document incorporated therein by reference or file any other
required document so that such Registration Statement will not contain an untrue
statement of a material fact or omit to state a material fact required to be
stated therein or necessary to make the statements therein not misleading, and
so that, as thereafter delivered to purchasers of the Shares being sold
thereunder, such prospectus will not contain an untrue statement of a material
fact or omit to state a material fact required to be stated therein or necessary
to make the statements therein, in light of the circumstances under which they
were made, not misleading; (ii) provide the Investor copies of any documents
filed pursuant to this Section 9.2(b)(i); and (iii) inform the Investor that the
Company has complied with its obligations in this Section 9.2(b)(i) (or that, if
the Company has filed a post-effective amendment to the Registration Statement
which has not yet been declared effective, the Company will notify the Investor
to that effect, will use commercially reasonable efforts to secure the
effectiveness of such post-effective amendment as promptly as possible and will
promptly notify the Investor pursuant to this Section 9.2(b)(i) when the
amendment has become effective).

                         (c) Subject to paragraph (d) below, in the event (i) of
any request by the SEC or any other federal or state governmental authority
during the period of effectiveness of the Registration Statement for amendments
or supplements to a Registration Statement or related prospectus or for
additional information; (ii) of the issuance by the SEC or any other federal or
state governmental authority of any stop order suspending the effectiveness of a
Registration Statement or the initiation of any proceedings for that purpose;
(iii) of the receipt by the Company of any notification with respect to the
suspension of the qualification or exemption from qualification of any of the
Shares

                                       12
<PAGE>   14
for sale in any jurisdiction or the initiation or threatening of any proceeding
for such purpose; or (iv) of any event or circumstance which, upon the advice of
its counsel, necessitates the making of any changes in the Registration
Statement or prospectus, or any document incorporated or deemed to be
incorporated therein by reference, so that, in the case of the Registration
Statement, it will not contain any untrue statement of a material fact or any
omission to state a material fact required to be stated therein or necessary to
make the statements therein not misleading, and that in the case of the
prospectus, it will not contain any untrue statement of a material fact or any
omission to state a material fact required to be stated therein or necessary to
make the statements therein, in the light of the circumstances under which they
were made, not misleading; then the Company shall deliver a certificate in
writing to the Investor (the "Suspension Notice") to the effect of the foregoing
and, upon receipt of such Suspension Notice, the Investor will refrain from
selling any Shares pursuant to the Registration Statement (a "Suspension") until
the Investor's receipt of copies of a supplemented or amended prospectus
prepared and filed by the Company, or until it is advised in writing by the
Company that the current prospectus may be used, and has received copies of any
additional or supplemental filings that are incorporated or deemed incorporated
by reference in any such prospectus. In the event of any Suspension, the Company
will use its commercially reasonable efforts to cause the use of the prospectus
so suspended to be resumed as soon as reasonably practicable. In addition to and
without limiting any other remedies (including, without limitation, at law or at
equity) available to the Investor, the Investor shall be entitled to specific
performance in the event that the Company fails to comply with the provisions of
this Section 9.2(c).

                         (d) Notwithstanding the foregoing paragraphs of this
Section 9.2, the Investor shall not be prohibited from selling the Shares under
the Registration Statement as a result of Suspensions on more than two occasions
of not more than thirty days each in any twelve month period, unless, in the
good faith judgment of the Company's Board of Directors, upon advice of counsel,
the sale of the Shares under the Registration Statement in reliance on this
Section 9.2(d) would be reasonably likely to cause a violation of the Securities
Act or the Exchange Act and result in liability to the Company.

                         (e) Provided that a Suspension is not then in effect,
the Investor may sell the Shares under the Registration Statement; provided,
however, that it arranges for delivery of a current prospectus to the transferee
of such Shares. Upon receipt of a request therefor, the Company has agreed to
provide an adequate number of current prospectuses to the Investor and to supply
copies to any other parties requiring such prospectuses.

                         (f) In the event of a sale of the Shares by the
Investor pursuant to the Registration Statement, the Investor must also deliver
to the Company's transfer agent, with a copy to the Company, a Certificate of
Subsequent Sale substantially in the form attached hereto as Exhibit B so that
the Shares may be properly transferred. Assuming timely delivery to the
Company's transfer agent of one or more stock certificates representing the
Shares in proper form for transfer and assuming compliance by the Investor with
the terms of this Agreement, the Company's transfer agent will issue and make
appropriate delivery of one or more stock certificates in the name of the buyer
so as to permit timely compliance by the Investor with applicable settlement
requirements.

                  9.3. Indemnification.

                         (a) Definitions. For the purpose of this Section 9.3:


                                       13
<PAGE>   15
                              (i) the term "Selling Stockholder" shall include
the Investor and each person, if any, who controls the Investor within the
meaning of Section 15 of the Securities Act, including any officer, director,
trustee or Affiliate of such Investor;

                              (ii) the term "Registration Statement" shall
include any final prospectus, exhibit, supplement or amendment included in or
relating to the Registration Statement referred to in Section 9.1; and

                              (iii) the term "untrue statement" shall include
any untrue statement or alleged untrue statement, or any omission or alleged
omission to state in the Registration Statement a material fact required to be
stated therein or necessary to make the statements therein, in the light of the
circumstances under which they were made, not misleading.

                         (b) Indemnification by the Company. The Company agrees
to indemnify and hold harmless the Selling Stockholder from and against any
losses, claims, damages or liabilities to which such Selling Stockholder may
become subject (under the Securities Act or otherwise) insofar as such losses,
claims, damages or liabilities (or actions or proceedings in respect thereof)
arise out of, or are based upon, (i) any untrue statement of a material fact
contained in the Registration Statement, or (ii) any failure by the Company to
fulfill any undertaking included in the Registration Statement, and the Company
will reimburse such Selling Stockholder for any reasonable legal or other
expenses reasonably incurred in investigating, defending or preparing to defend
any such action, proceeding or claim, or preparing to defend any such action,
proceeding or claim; provided, however, that the Company shall not be liable in
any such case to the extent that such loss, claim, damage or liability arises
out of, or is based upon, an untrue statement made in such Registration
Statement in reliance upon and in conformity with written information furnished
to the Company by or on behalf of the Selling Stockholder specifically for use
in preparation of the Registration Statement or the failure of the Selling
Stockholder to comply with its covenants and agreements contained in Section 9.2
hereof respecting the sale of the Shares or any statement or omission in any
prospectus that is corrected in any subsequent prospectus that was delivered to
the Investor prior to the pertinent sale or sales by the Investor; provided
however, that the Selling Stockholder shall be entitled to be indemnified in any
such case for any statement or alleged statement in or omission or alleged
omission from such Registration Statement, preliminary prospectus, final
prospectus or summary prospectus contained therein, or any amendment or
supplement thereto, in which such statement or omission has been corrected, in
writing, by the Investor and delivered to the Company at least ten (10) days
before the sale or sales from which such loss occurred. The Company shall
reimburse the Selling Stockholder for the amounts provided for herein on demand
as such expenses are incurred.

                         (c) Indemnification by the Investor. The Investor
agrees to indemnify and hold harmless the Company (and each person, if any, who
controls the Company within the meaning of Section 15 of the Securities Act,
each officer of the Company who signs the Registration Statement and each
director of the Company) from and against any losses, claims, damages or
liabilities to which the Company (or any such officer, director or controlling
person) may become subject (under the Securities Act or otherwise), insofar as
such losses, claims, damages or liabilities (or actions or proceedings in
respect thereof) arise out of, or are based upon, (i) any failure to comply with
the covenants and agreements contained in Section 9.2 hereof respecting the sale
of the Shares, or (ii) any untrue statement of a material fact contained in the
Registration Statement if such untrue statement was made in reliance upon and in
conformity with written information furnished by or on behalf of the Investor
specifically for use in preparation of the Registration Statement, and the
Investor will reimburse the Company (or such officer, director or controlling
person), as the case may be, for any legal or other

                                       14
<PAGE>   16
expenses reasonably incurred in investigating, defending or preparing to defend
any such action, proceeding or claim; provided, however, that the Investor's
obligation to indemnify the Company shall be limited to the net amount received
by the Investor from the sale of the Shares; and further provided however, that
the Selling Stockholder shall have no obligation to indemnify the Company in any
such case for any statement or alleged statement in or omission or alleged
omission from such Registration Statement, preliminary prospectus, final
prospectus or summary prospectus contained therein, or any amendment or
supplement thereto, in which such statement or omission has been corrected, in
writing, by the Investor and delivered to the Company at least ten (10) days
before the sale or sales from which such loss occurred.

                         (d) Notice of Claims, Etc. Promptly after receipt by
any indemnified person of a notice of a claim or the beginning of any action in
respect of which indemnity is to be sought against an indemnifying person
pursuant to this Section 9.3, such indemnified person shall notify the
indemnifying person in writing of such claim or of the commencement of such
action, but the omission to so notify the indemnifying party will not relieve it
from any liability which it may have to any indemnified party under this Section
9.3 (except to the extent that such omission materially and adversely affects
the indemnifying party's ability to defend such action) or from any liability
otherwise than under this Section 9.3. Subject to the provisions hereinafter
stated, in case any such action shall be brought against an indemnified person,
the indemnifying person shall be entitled to participate therein, and, to the
extent that it shall elect by written notice delivered to the indemnified party
promptly after receiving the aforesaid notice from such indemnified party, shall
be entitled to assume the defense thereof, with counsel reasonably satisfactory
to such indemnified person. After notice from the indemnifying person to such
indemnified person of its election to assume the defense thereof, such
indemnifying person shall not be liable to such indemnified person for any legal
expenses subsequently incurred by such indemnified person in connection with the
defense thereof; provided, however, that if there exists or shall exist a
conflict of interest that would make it inappropriate, in the opinion of counsel
to the indemnified person, for the same counsel to represent both the
indemnified person and such indemnifying person or any Affiliate or associate
thereof, the indemnified person shall be entitled to retain its own counsel at
the expense of such indemnifying person; provided, however, that no indemnifying
person shall be responsible for the fees and expenses of more than one separate
counsel (together with appropriate local counsel) for all indemnified parties.
In no event shall any indemnifying person be liable in respect of any amounts
paid in settlement of any action unless the indemnifying person shall have
approved the terms of such settlement; provided, however, that such consent
shall not be unreasonably withheld. No indemnifying person shall, without the
prior written consent of the indemnified person, effect any settlement of any
pending or threatened proceeding in respect of which any indemnified person is
or could have been a party and indemnification could have been sought hereunder
by such indemnified person, unless such settlement includes an unconditional
release of such indemnified person from all liability on claims that are the
subject matter of such proceeding.

                         (e) Contribution. If the indemnification provided for
in this Section 9.3 is unavailable to or insufficient to hold harmless an
indemnified party under paragraph (b) or (c) above in respect of any losses,
claims, damages or liabilities (or actions or proceedings in respect thereof)
referred to therein, then each indemnifying party shall contribute to the amount
paid or payable by such indemnified party as a result of such losses, claims,
damages or liabilities (or actions in respect thereof) in such proportion as is
appropriate to reflect the relative fault of the Company on the one hand and the
Investor, as well as any other selling stockholders under such registration
statement on the other in connection with the statements or omissions or other
matters which resulted in such losses, claims, damages or liabilities (or
actions in respect thereof), as well as any other relevant equitable
considerations. The relative fault shall be determined by reference to, among
other things, in the case

                                       15
<PAGE>   17
of an untrue statement, whether the untrue statement relates to information
supplied by the Company on the one hand or an Investor or other selling
stockholder on the other and the parties' relative intent, knowledge, access to
information and opportunity to correct or prevent such untrue statement. The
Company and the Investor agree that it would not be just and equitable if
contribution pursuant to this paragraph (e) were determined by pro rata
allocation (even if the Investor and other selling stockholders were treated as
one entity for such purpose) or by any other method of allocation which does not
take into account the equitable considerations referred to above in this
paragraph (e). The amount paid or payable by an indemnified party as a result of
the losses, claims, damages or liabilities (or actions in respect thereof)
referred to above in this paragraph (e) shall be deemed to include any legal or
other expenses reasonably incurred by such indemnified party in connection with
investigating or defending any such action or claim. Notwithstanding the
provisions of this paragraph (e), the Investor shall not be required to
contribute any amount in excess of the amount by which the net amount received
by the Investor from the sale of the Shares to which such loss relates exceeds
the amount of any damages which such Investor has otherwise been required to pay
by reason of such untrue statement. No person guilty of fraudulent
misrepresentation (within the meaning of Section 11(f) of the Securities Act)
shall be entitled to contribution from any person who was not guilty of such
fraudulent misrepresentation.

         9.4. Lockup Agreement. The Investor agrees, for a period of two years
following the Closing, and if the Investor then beneficially owns at least 3% of
the outstanding Common Stock of the Company (adjusted as hereinafter provided),
in connection with any primary underwritten registration of the Company's
securities (other than registrations on Forms S-4 or S-8 or comparable forms),
upon the request of the underwriters managing the offering, not to sell, make
any short sale of, pledge, grant any option for the purchase of or otherwise
dispose of any Shares (other than those included in the registration) without
the prior written consent of such underwriters during the seven days prior to
and during the 90-day period beginning on the effective date of such
registration as the underwriters may specify; provided that all of the following
similarly agree: (a) all officers of the Company, (b) all directors of the
Company, and (c) all holders of more than one percent of the outstanding capital
stock of the Company who either (i) purchased such stock from the Company in
connection with entering into a collaborative arrangement or (ii) purchased such
stock from the Company in a private placement subsequent to the date of this
Agreement. The 3% threshold shall be appropriately adjusted to reflect any stock
repurchase, or similar transaction, by the Company which shall have the effect
of reducing the number of outstanding shares of the Company's Common Stock.

         10. Rule 144. The Company covenants that it will file the reports
required to be filed by it under the Securities Act and the Exchange Act and the
rules and regulations adopted by the SEC thereunder (or, if the Company is not
required to file such reports, it will, upon the request of the Investor, made
after the first anniversary of the Closing Date, make publicly available such
information as necessary to permit sales pursuant to Rule 144 under the
Securities Act), and it will take such further action as the Investor may
reasonably request, all to the extent required from time to time to enable the
Investor to sell the Shares purchased hereunder without registration under the
Securities Act within the limitation of the exemptions provided by (a) Rule 144
under the Securities Act, as such Rule may be amended from time to time, or (b)
any similar rule or regulation hereafter adopted by the SEC. Upon the request of
the Investor, the Company will deliver to it a written statement as to whether
the Company has complied with such information and requirements.


                                       16
<PAGE>   18
         11. Additional Covenant.

                  11.1. The Investor agrees that for a period beginning on the
date of this Agreement and ending on the earlier of (a) the occurrence of a
Strategic Event (as defined in Section 11.2), or (b) three years from the date
of this Agreement, without the prior written consent of the Company, neither the
Investor nor any Affiliate of the Investor (regardless of whether such person or
entity is an Affiliate on the date of this Agreement) will (i) acquire, offer to
acquire, or agree to acquire, directly, or indirectly through Affiliates or
through a Group (as defined in Section 11.3), by purchase or otherwise, any
Voting Securities or rights or options to acquire any Voting Securities, if such
acquisition would result in aggregate voting power in the election of directors
of the Company of all Voting Securities then owned by the Investor, together
with any Affiliate of the Investor or any Group in which the Investor or any
Affiliate of the Investor shall then be participating, of greater than 4.95% of
such total combined voting power of all Voting Securities then outstanding, or
(ii) make, or in any way participate, directly or indirectly through Affiliates
or through membership in a Group, in any "solicitation" of "proxies" to vote (as
such terms are defined in Rule 14a-1 under the Exchange Act) any Voting
Securities.

                  11.2. The term "Strategic Event" as used in Section 11.1 shall
mean any of the following events:

                         (a) commencement by a Third Party (as defined in
Section 11.4) of a tender offer or exchange offer for a majority of the
outstanding Voting Securities;

                         (b) filing by a Third Party with the SEC of a proxy
statement for use in connection with the solicitation of proxies in opposition
to management's slate of candidates for election to the Company's board of
directors;

                         (c) filing by a Third Party with the SEC of a Schedule
13D under the Exchange Act, if such Third Party has acquired or holds the
securities of the Company with a purpose or effect of changing or influencing
control of the Company, or in connection with or as a participant in any
transaction having that purpose or effect; or

                         (d) public announcement by the Company of an agreement
with a Third Party to merge or consolidate the Company with a Third Party in a
transaction that would result in an aggregate change in the ownership or control
of more than 50% of the Voting Securities, or the sale of all or substantially
all of the Company's assets to a Third Party.

                  11.3. The term "Group" shall mean two or more persons (other
than Roche or any Affiliate of Roche) who agree to act together for the purpose
of acquiring, holding, voting or disposing of Voting Securities.

                  11.4. The term "Third Party" shall mean a Person other than:

                         (a) the Company or any Affiliate of the Company as of
the date of this Agreement;

                         (b) the Investor or any Affiliate of the Investor; and

                         (c) Roche or any Roche Affiliate.


                                       17
<PAGE>   19
                  11.5. In the event a Strategic Event occurs and the Company
has knowledge of it, the Company shall provide the Investor with written notice
of such Strategic Event within three Business Days of its occurrence (or within
three Business Days of the Company's awareness of its occurrence, whichever is
later). For purposes of this Section 11.5, "Business Day" shall mean any day on
which the New York Stock Exchange, Inc. is open for trading.

         12. Termination. Either party may terminate this Agreement, upon ten
days' prior written notice to the other party if the FTC does not approve the
transactions contemplated by this Agreement and the Collaboration Agreements
under the Hart-Scott-Rodino Act or if all applicable waiting periods, requests
for information (and any extensions thereof) under the Hart-Scott-Rodino Act
have not expired or otherwise been terminated by June 1, 2001, in which case
this Agreement shall forthwith become void and there shall be no liability or
obligation on the part of the Company or Genentech.

         13. Notices. All notices, requests, consents and other communications
hereunder shall be in writing, shall be mailed (a) if within domestic United
States by first-class registered or certified airmail, or nationally recognized
overnight express courier, postage prepaid, or by facsimile, or (b) if delivered
from outside the United States, by International Federal Express or facsimile,
and shall be deemed given (i) if delivered by first-class registered or
certified mail domestic, three business days after so mailed, (ii) if delivered
by nationally recognized overnight carrier, one business day after so mailed,
(iii) if delivered by International Federal Express, two business days after so
mailed, or (iv) if delivered by facsimile, upon electronic confirmation of
receipt and shall be delivered as addressed as follows:

                           (a)      if to the Company, to:

                                    Robert L. Van Nostrand
                                    Vice President and Chief Financial Officer
                                    OSI Pharmaceuticals, Inc.
                                    106 Charles Lindbergh Boulevard
                                    Uniondale, NY 11553
                                    Telecopy:  (516) 222-0964

                                    with a copy to:

                                    Spencer W. Franck, Jr., Esquire
                                    Saul Ewing LLP
                                    Centre Square West
                                    1500 Market Street, 38th Floor
                                    Philadelphia, PA 19102
                                    Phone:  (215) 972-1955
                                    Telecopy:  (215) 972-1938

                           (b)      if to the Investor, at its address on the
                                    signature page hereto, or at such other
                                    address or addresses as may have been
                                    furnished to the Company in writing,


                                       18
<PAGE>   20
                                    with a copy to:

                                    Corporate Secretary
                                    Genentech, Inc.
                                    1 DNA Way South
                                    San Francisco, CA 94080-4990
                                    Phone:  (650) 225-1672
                                    Telecopy: (650) 225-8654

         14. Changes. This Agreement may not be modified or amended except
pursuant to an instrument in writing signed by the Company and the Investor.

         15. Headings. The headings of the various sections of this Agreement
have been inserted for convenience of reference only and shall not be deemed to
be part of this Agreement.

         16. Severability. In case any provision contained in this Agreement
should be invalid, illegal or unenforceable in any respect, the validity,
legality and enforceability of the remaining provisions contained herein shall
not in any way be affected or impaired thereby.

         17. Successors and Assigns. This Agreement may be assigned by the
Investor at any time following the Closing. Without the prior written consent of
the Investor, but after notice duly given and in compliance with this Agreement,
the Company may assign its rights and delegate its duties hereunder to any
successor-in-interest corporation in the event of a merger or consolidation of
the Company with or into another corporation, or any merger or consolidation of
another corporation with or into the Company that results directly or indirectly
in an aggregate change in the ownership or control of more than 50% of the
voting rights of the equity securities of the Company, or the sale of all or
substantially all of the Company's assets. The terms and conditions of this
Agreement shall inure to the benefit of and be binding upon the respective
permitted successors and assigns of the parties. Nothing in this Agreement,
express or implied, is intended to confer upon any party other than the parties
hereto or their respective successors and assigns any rights, remedies,
obligations, or liabilities under or by reason of this Agreement, except as
expressly provided in this Agreement.

         18. Further Assurances. The parties shall execute and deliver all such
further instruments and documents and take all such other actions as may
reasonably be required to carry out the transactions contemplated hereby and to
evidence the fulfillment of the agreements herein contained.

         19. Governing Law. This Agreement shall be governed by, and construed
in accordance with, the internal laws of the State of Delaware, without giving
effect to the principles of conflicts of law.

         20. Counterparts. This Agreement may be executed in counterparts, each
of which shall constitute an original, but all of which, when taken together,
shall constitute but one instrument, and shall become effective when one or more
counterparts have been signed by each party hereto and delivered to the other
parties.

                            [Signatures on next page]


                                       19
<PAGE>   21
         IN WITNESS WHEREOF, the parties have executed this Agreement as of the
date first written above.

                             OSI PHARMACEUTICALS, INC.

                             By:  /s/ COLIN GODDARD
                                  --------------------------------------

                             Title:  Chairman and CEO
                                     ---------------------------------------



                             GENENTECH, INC.


                             By:  /s/  ARTHUR D. LEVINSON, PH.D.
                                  ---------------------------------------

                             Print Name:  Arthur D. Levinson, Ph.D.
                                          ------------------------------------

                             Title:  Chairman and Chief Executive Officer
                                     -------------------------------------------

                             Address: 1 DNA Way, South San Francisco, CA 94080
                                      ----------------------------------------

                             Tax ID No.:  94 2347624
                                          ----------------------------------

                             Contact name:  Myrtel Potter
                                            ----------------------------------

                             Telephone:  650-225-1000
                                         ------------------------------------

                             Name in which shares should registered
                               (if different):

                             -------------------------------------------------

                                       20
<PAGE>   22
                                    EXHIBIT A

                      FORM OF OPINION OF COMPANY'S COUNSEL

         The opinion shall include the following matters:

         1. Organization. The Company and each of its subsidiaries is duly
organized and validly existing in good standing under the laws of the
jurisdiction of its organization. Each of the Company and its subsidiaries has
full power and authority to own, operate and occupy its properties and to
conduct its business as presently conducted. Each of the Company and its
subsidiaries is registered or qualified to do business and is in good standing
in each jurisdiction in which it owns or leases property or transacts business
and where the failure to be so qualified would have a Material Adverse Effect.
To such counsel's knowledge, no proceeding has been instituted in any such
jurisdiction, revoking, limiting or curtailing, or seeking to revoke, limit or
curtail, such power and authority or qualification.

         2. Due Authorization and Valid Issuance. The Company has all requisite
corporate power and authority to execute, deliver and perform its obligations
under this Agreement. The Agreement has been duly authorized and validly
executed and delivered by the Company and constitutes a legal, valid and binding
agreement of the Company enforceable against the Company in accordance with its
terms, except as (a) rights to indemnity and contribution may be limited by
state or federal securities laws or the public policy underlying such laws, (b)
enforceability may be limited by applicable bankruptcy, insolvency,
reorganization, moratorium or similar laws affecting creditors' and contracting
parties' rights generally, and (c) enforceability may be subject to general
principles of equity (regardless of whether such enforceability is considered in
a proceeding in equity or at law). The Shares have been duly authorized, and
duly and validly issued, fully paid and nonassessable and free and clear of all
encumbrances and restrictions on transfer, except for restrictions on transfer
imposed by the Agreement or by applicable federal or state securities laws.

         3. Non-Contravention. The execution and delivery of the Agreement, the
issuance and sale of the Shares to be sold by the Company under the Agreement,
the fulfillment of the terms of the Agreement and the consummation of the
transactions contemplated thereby will not (a) conflict with or constitute a
violation of, or default (with the passage of time or otherwise) under (x) the
charter, by-laws or other organizational documents of the Company or any
subsidiary or, (y) to such counsel's knowledge, (i) any material bond,
debenture, note or other evidence of indebtedness, lease, contract, indenture,
mortgage, deed of trust, loan agreement, joint venture or other agreement or
instrument to which the Company or any of its subsidiaries is a party or by
which it or any of its subsidiaries or their respective properties are bound, or
(ii) any law, administrative regulation, ordinance or order of any court or
governmental agency, arbitration panel or authority applicable to the Company or
any subsidiary or their respective properties, or (b) to such counsel's
knowledge, result in the creation or imposition of any lien, encumbrance, claim,
security interest or restriction whatsoever upon any of the material properties
or assets of the Company or any subsidiary or an acceleration of indebtedness
pursuant to any obligation, agreement or condition contained in any material
bond, debenture, note or any other evidence of indebtedness or any material
indenture, mortgage, deed of trust or any other agreement or instrument to which
the Company or any subsidiary is a party or by which any of them is bound or to
which any of the property or assets of the Company or any subsidiary is subject.
To such counsel's knowledge, no consent, approval, authorization or other order
of, or registration, qualification or filing with, any regulatory body,
administrative agency, or other governmental body in the United States or any
other Person is required for the execution and delivery of the Agreement and the
valid issuance and

                                       A-1
<PAGE>   23
sale of the Shares, other than such as have been made or obtained, and except
for any securities filings or notifications required to be made after the
Closing under federal or state securities laws.

         4. Authorized Capitalization. The authorized capital of the Company
consists of 50,000,000 shares of Common Stock, par value $.01 per share, and
5,000,000 shares of Preferred Stock, $.01 par value. None of the shares of
Preferred Stock are issued and outstanding.

         5. Securities Act. Assuming the accuracy of the representations and
warranties made by the Investor in the Agreement, the offer and sale of the
Shares to the Investor is exempt from the registration requirements of the
Securities Act of 1933, as amended, by virtue of Section 4(2) thereof.

         6. Legal Proceedings. To such counsel's knowledge, there is no material
legal or governmental proceeding pending or threatened to which the Company or
any subsidiary is or may be a party or of which the business or property of the
Company or any subsidiary is subject.

         7. Nasdaq Compliance. The Company's Common Stock is registered pursuant
to Section 12(g) of the Exchange Act and is listed on the Nasdaq National
Market, and, to such counsel's knowledge, the Company has taken no action
designed to, or likely to have the effect of, terminating the registration of
the Common Stock under the Exchange Act or de-listing the Common Stock from the
Nasdaq National Market, nor, to such counsel's knowledge, has the Company
received any notification that the SEC or the NASD is contemplating terminating
such registration or listing.

                                      A-2
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                                    EXHIBIT B

                         CERTIFICATE OF SUBSEQUENT SALE

Bank of New York
101 Barclay Street
New York, NY  10286

         RE:      Sale of Shares of Common Stock of OSI Pharmaceuticals, Inc.
                  (the "Company") pursuant to the Company's Prospectus dated
                  _____________, ____ (the "Prospectus")

Dear Sir/Madam:

         The undersigned hereby certifies, in connection with the sale of shares
of Common Stock of the Company included in the table of Selling Stockholders in
the Prospectus, that the undersigned has sold the shares pursuant to the
Prospectus and in a manner described under the caption "Plan of Distribution" in
the Prospectus and that such sale complies with all applicable securities laws,
applicable to the undersigned, including, without limitation, the Prospectus
delivery requirements of the Securities Act of 1933, as amended.

Selling Stockholder (the beneficial owner): ____________________________________

Record Holder (e.g., if held in name of nominee):______________________________

Restricted Stock Certificate No.(s): __________________________________________

Number of Shares Sold: _______________________________________

Date of Sale: _________________________________________________________

         In the event that you receive a stock certificate(s) representing more
shares of Common Stock than have been sold by the undersigned, then you should
return to the undersigned a newly issued certificate for such excess shares in
the name of the Record Holder and BEARING A RESTRICTIVE LEGEND. Further, you
should place a stop transfer on your records with regard to such certificate.

                                                  Very truly yours,


Dated:  _____________________                     By:__________________________

                                                  Print Name:___________________

                                                  Title: ______________________


cc:      Mr. Robert L. Van Nostrand
         Vice President and Chief Financial Officer
         OSI Pharmaceuticals, Inc.
         106 Charles Lindbergh Boulevard
         Uniondale, NY 11553

                                      B-1