AGREEMENT AND PLAN OF MERGER

BY AND AMONG

IDEC PHARMACEUTICALS CORPORATION,

BRIDGES MERGER CORPORATION

AND

BIOGEN, INC.

DATED AS OF JUNE 20, 2003

AGREEMENT AND PLAN OF MERGER

TABLE OF CONTENTS

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AGREEMENT AND PLAN OF MERGER

        THIS AGREEMENT AND PLAN OF MERGER (this "Agreement") is made and entered into as of June 20, 2003, by and among IDEC PHARMACEUTICALS CORPORATION, a Delaware corporation ("IDEC"), BRIDGES MERGER CORPORATION, a Massachusetts corporation and a direct wholly owned subsidiary of IDEC ("Merger Sub"), and BIOGEN, INC., a Massachusetts corporation ("Biogen").

W I T N E S S E T H:

        WHEREAS, the respective Boards of Directors of IDEC, Merger Sub and Biogen have deemed it advisable and in the best interests of their respective corporations and stockholders that IDEC and Biogen engage in a business combination in a merger of equals in order to advance their respective long-term strategic business interests; and

        WHEREAS, in furtherance thereof, the Boards of Directors of each of IDEC, Merger Sub and Biogen have approved this Agreement and the merger of Merger Sub with and into Biogen with Biogen continuing as the surviving corporation (the "Merger"), upon the terms and subject to the conditions set forth in this Agreement and in accordance with the provisions of the Massachusetts Business Corporation Law (the "MBCL"); and

        WHEREAS, the Board of Directors of Biogen has determined to recommend to its stockholders the approval and adoption of this Agreement and the Merger; and

        WHEREAS, the Board of Directors of IDEC has determined to recommend to its stockholders approval of the IDEC Charter Amendment (as defined in Section 1.8) and the issuance of shares of IDEC Common Stock (as defined in Section 2.1(a)) in connection with the Merger (the "Share Issuance"); and

        WHEREAS, IDEC, as the sole stockholder of Merger Sub, has approved this Agreement and the Merger; and

        WHEREAS, for United States federal income tax purposes, it is intended that the Merger shall qualify as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code of 1986, as amended (the "Code"), and this Agreement is intended to be, and is hereby, adopted as a plan of reorganization within the meaning of Section 368 of the Code.

        NOW, THEREFORE, in consideration of the foregoing and the representations, warranties, covenants and agreements set forth herein, and for other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, the parties agree as follows:

ARTICLE I

THE MERGER

        SECTION 1.1    The Merger.    Upon the terms and subject to the conditions set forth in this Agreement, and in accordance with the MBCL, at the Effective Time (as defined in Section 1.3), Merger Sub shall be merged with and into Biogen, the separate corporate existence of Merger Sub shall cease and Biogen shall continue as the surviving corporation in the Merger (the "Surviving Corporation") and shall succeed to and assume all the property, rights, privileges, powers and franchises of Merger Sub in accordance with the MBCL.

        SECTION 1.2    Closing.    The closing of the Merger (the "Closing") shall take place at 10:00 a.m., Boston time, on a date to be specified by the parties, which shall be no later than the second business day after satisfaction or waiver of all of the conditions set forth in Article VI (other than delivery of items to be delivered at the Closing and other than those conditions that by their nature are to be

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satisfied at the Closing, it being understood that the occurrence of the Closing shall remain subject to the delivery of such items and the satisfaction or waiver of such conditions at the Closing) at the offices of Skadden, Arps, Slate, Meagher & Flom LLP, One Beacon Street, Boston, Massachusetts, unless another time, date or place is agreed to in writing by the parties hereto. The date on which the Closing occurs is referred to herein as the "Closing Date."

        SECTION 1.3    Effective Time.    Subject to the terms and conditions of this Agreement, as soon as practicable on the Closing Date, the parties shall cause the Merger to be consummated by filing articles of merger executed in accordance with the relevant provisions of the MBCL (the "Articles of Merger") with the Secretary of State of the Commonwealth of Massachusetts (the "Secretary of State") and shall make all other filings or recordings required under the MBCL. The Merger shall become effective at such time as the Articles of Merger are duly filed with the Secretary of State, or at such subsequent date or time as IDEC and Biogen shall agree and specify in the Articles of Merger. The time at which the Merger becomes effective is referred to herein as the "Effective Time."

        SECTION 1.4    Effects of The Merger.    At the Effective Time, the Merger shall have the effects set forth in this Agreement and in the applicable provisions of the MBCL.

        SECTION 1.5    Purposes of the Surviving Corporation.    The purposes of the Surviving Corporation shall be as set forth in Article 2 of the articles of organization of the Surviving Corporation.

        SECTION 1.6    Organizational Documents of the Surviving Corporation.    At the Effective Time, the Biogen Charter (as defined in Section 3.1(a)(ii)) shall be amended and restated in its entirety to be identical to the articles of organization of Merger Sub, as in effect immediately prior to the Effective Time, until thereafter amended in accordance with the MBCL and as provided in such articles of organization; provided, however, that at the Effective Time, Article I of the articles of organization of the Surviving Corporation shall be amended and restated in its entirety to read as follows: "The name of the corporation is Biogen, Inc." After the Effective Time, the authorized capital stock of the Surviving Corporation shall consist of 1,000 shares of common stock, par value $.01 per share. At the Effective Time, the Biogen By-Laws (as defined in Section 3.1(a)(ii)) shall be amended and restated in their entirety to be identical to the by-laws of Merger Sub, as in effect immediately prior to the Effective Time, until thereafter amended in accordance with the MBCL and as provided in such by-laws.

        SECTION 1.7    Directors and Officers of the Surviving Corporation.    The initial directors of the Surviving Corporation shall be the IDEC Designated Directors (as defined in Section 5.14) and the Biogen Designated Directors (as defined in Section 5.14), until their respective successors are duly elected or appointed and qualified. The initial officers of the Surviving Corporation shall be the officers of Biogen immediately prior to the Effective Time, until their respective successors are duly appointed.

        SECTION 1.8    IDEC Charter Amendment.    Immediately prior to the Effective Time, and subject to the terms and conditions of this Agreement and receipt of IDEC Stockholder Approval (as defined in Section 3.2(c)(iii)), pursuant to the applicable provisions of Delaware General Corporation Law, IDEC shall cause the IDEC Charter (as defined in Section 3.2(a)(ii)) to be amended (the "IDEC Charter Amendment") to (i) change the name of IDEC to "BIOGEN IDEC, INC." and (ii) increase the number of authorized shares of IDEC Common Stock to one billion (1,000,000,000) shares, by filing a Certificate of Amendment substantially in the form of Exhibit A hereto (the "IDEC Certificate of Amendment") with the Secretary of State of the State of Delaware in accordance with applicable provisions of the Delaware General Corporation Law.

        SECTION 1.9    Alternative Structure.    Biogen and IDEC may mutually agree to revise the structure of the Merger provided for herein at any time prior to receipt of either Biogen Stockholder Approval (as defined in Section 3.1(c)(iii)) or IDEC Stockholder Approval, or at any time thereafter if, with appropriate disclosure, any required further approval of the revised structure is obtained from the

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stockholders of Biogen and IDEC, as applicable; provided, however, that under any such revised structure the Merger would qualify as a reorganization within the meaning of Section 368(a) of the Code. Biogen and IDEC agree to consider in good faith the request of the other party to revise the structure of the Merger from that set forth herein.

ARTICLE II

EFFECTS OF THE MERGER; EXCHANGE OF CERTIFICATES

        SECTION 2.1    Effect on Capital Stock.    Subject to the terms and conditions of this Agreement, at the Effective Time, by virtue of the Merger and without any action on the part of IDEC, Merger Sub, Biogen or the holders of any shares of common stock, par value $0.01 per share, of Biogen (together with any associated Biogen Rights (as defined in Section 3.1(b)(i)), "Biogen Common Stock"):

        (a)    Conversion of Biogen Common Stock.    Each share of Biogen Common Stock issued and outstanding immediately prior to the Effective Time, other than any shares of Biogen Common Stock to be canceled pursuant to Section 2.1(c) or, subject to Section 2.1(g), held by a Dissenting Holder (as defined in Section 2.1(g)(i)), shall be automatically converted into the right to receive 1.150 (the "Exchange Ratio") fully paid and nonassessable shares of common stock, par value $0.0005 per share ("IDEC Common Stock"), of IDEC (together with any associated IDEC Rights (as defined in Section 3.2(b)(i)), the "Merger Consideration")) upon surrender of the Certificate (as defined in Section 2.2(b)) which immediately prior to the Effective Time represented such share of Biogen Common Stock in the manner provided in Section 2.2(b) (or, in the case of a lost, stolen or destroyed certificate, Section 2.2(i)). As a result of the Merger, at the Effective Time, each holder of a Certificate shall cease to have any rights with respect thereto, except (i) the right to receive the Merger Consideration payable in respect of the shares of Biogen Common Stock represented by such Certificate immediately prior to the Effective Time, any cash in lieu of fractional shares payable pursuant to Section 2.1(e) and any dividends or other distributions payable pursuant to Section 2.2(c) , all to be issued or paid, without interest, in consideration therefor upon the surrender of such Certificate in accordance with Section 2.2(b) or (ii) in the case of a Dissenting Holder, the rights set forth in Section 2.1(g).

        (b)    Capital Stock of Merger Sub.    Each issued and outstanding share of common stock, par value $0.01 per share, of Merger Sub shall be converted into one fully paid and nonassessable share of common stock, par value $0.01 per share, of the Surviving Corporation.

        (c)    Cancellation of Treasury Shares.    Each share of Biogen Common Stock held in the treasury of Biogen, if any, shall automatically be extinguished without any conversion, and no consideration shall be delivered in respect thereof.

        (d)    Biogen Options and Employee Stock Purchase Plans.    At the Effective Time, (i) all options to purchase Biogen Common Stock (each, a "Biogen Option") issued and outstanding under each Biogen Stock Plan (as defined in Section 3.1(b)(i)) shall be assumed by IDEC in accordance with Section 5.12(a) and (ii) all rights outstanding under Biogen's 1983 Employee Stock Purchase Plan, as amended and restated, and the 1994 International Employee Stock Purchase Plan, as amended and restated (collectively, the "Biogen Purchase Plans"), shall be treated as set forth in Section 5.12 (b).

        (e)    Fractional Shares.    No fraction of a share of IDEC Common Stock will be issued by virtue of the Merger, but in lieu thereof each holder of shares of Biogen Common Stock who would otherwise be entitled to a fraction of a share of IDEC Common Stock (after aggregating all fractional shares of IDEC Common Stock that otherwise would be received by such holder) shall, upon surrender of such holder's Certificate(s), receive from IDEC an amount of cash (rounded to the nearest whole cent), without interest, equal to the product of: (i) such fraction, multiplied by

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(ii) the average closing price of one share of IDEC Common Stock for the ten (10) most recent trading days that IDEC Common Stock has traded ending on the trading day one day prior to the Effective Time, as reported on the Nasdaq National Market.

        (f)    Adjustments to Exchange Ratio.    The Exchange Ratio shall be adjusted to reflect fully the appropriate effect of any stock split, reverse stock split, stock dividend (including any dividend or distribution of securities convertible into IDEC Common Stock or Biogen Common Stock), reorganization, recapitalization, reclassification or other like change with respect to IDEC Common Stock or Biogen Common Stock having a record date on or after the date hereof and prior to the Effective Time.

        (g)    Dissenter's Rights.    

        (i)    No conversion under Section 2.1(a) hereof shall occur with respect to the shares of Biogen Common Stock held by a Dissenting Holder; provided, however, that each share of Biogen Common Stock outstanding immediately prior to the Effective Time and held by a Dissenting Holder who shall, after the Effective Time, withdraw his demand for appraisal or lose his right of appraisal, in either case pursuant to the applicable provisions of the MBCL, shall be deemed to be converted, as of the Effective Time, into the Merger Consideration as set forth in Section 2.1(a) hereof. The term "Dissenting Holder" shall mean a holder of Biogen Common Stock who has demanded appraisal rights in compliance with the applicable provisions of the MBCL concerning the right of such holder to dissent from the Merger and demand appraisal of such holder's shares of Biogen Common Stock.

        (ii)   Any Dissenting Holder (A) who (1) files with Biogen a written objection to the Merger before the Biogen Stockholders' Meeting (as defined in Section 5.1(b)) and (2) states in such objection that he intends to demand payment for his shares if the Merger is consummated and (B) whose shares are not voted in favor of the Merger shall be entitled to demand payment from Biogen for his shares of Biogen Common Stock and an appraisal of the value thereof, in accordance with the provisions of Sections 86 through 98 of the MBCL.

        SECTION 2.2    Exchange of Shares and Certificates.    

        (a)    Exchange Agent.    At or prior to the Effective Time, IDEC shall engage an institution reasonably satisfactory to Biogen to act as exchange agent in connection with the Merger (the "Exchange Agent"), pursuant to an agreement reasonably satisfactory to Biogen. At the Effective Time, IDEC shall deposit with the Exchange Agent, in trust for the benefit of the holders of shares of Biogen Common Stock immediately prior to the Effective Time, certificates representing the shares of IDEC Common Stock issuable pursuant to Section 2.1(a). In addition, IDEC shall make available by depositing with the Exchange Agent, as necessary from time to time after the Effective Time as needed, cash in an amount sufficient to make the payments in lieu of fractional shares pursuant to Section 2.1(e) and any dividends or distributions to which holders of shares of Biogen Common Stock may be entitled pursuant to Section 2.2(c). All cash and IDEC Common Stock deposited with the Exchange Agent shall hereinafter be referred to as the "Exchange Fund."

        (b)    Exchange Procedures.    Promptly after the Effective Time, IDEC shall cause the Exchange Agent to mail to each holder of record of a certificate or certificates which immediately prior to the Effective Time represented outstanding shares of Biogen Common Stock (the "Certificates"), which at the Effective Time were converted into the right to receive the Merger Consideration pursuant to Section 2.1 hereof, (i) a letter of transmittal (which shall specify that delivery shall be effected, and risk of loss and title to the Certificates shall pass only upon delivery of the Certificates to the Exchange Agent and shall be in such form and have such other provisions as IDEC may reasonably specify) and (ii) instructions for use in effecting the surrender of the Certificates in exchange for certificates representing whole shares of IDEC Common Stock,

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cash in lieu of any fractional shares pursuant to Section 2.1(e) and any dividends or other distributions payable pursuant to Section 2.2(c). Upon surrender of Certificates for cancellation to the Exchange Agent or to such other agent or agents as may be appointed by IDEC, together with such letter of transmittal, duly completed and validly executed in accordance with the instructions thereto, and such other documents as may reasonably be required by the Exchange Agent, the holder of such Certificates shall be entitled to receive in exchange therefor a certificate representing that number of whole shares of IDEC Common Stock (after taking into account all Certificates surrendered by such holder) to which such holder is entitled pursuant to Section 2.1 (which shall be uncertificated book entry form unless a physical certificate is requested), payment in lieu of fractional shares which such holder is entitled to receive pursuant to Section 2.1(e) and any dividends or distributions payable pursuant to Section 2.2(c), and the Certificates so surrendered shall forthwith be canceled. In the event of a transfer of ownership of Biogen Common Stock which is not registered in the transfer records of Biogen, a certificate representing the proper number of shares of IDEC Common Stock may be issued to a Person (as defined in Section 8.3(o)) other than the Person in whose name the Certificate so surrendered is registered, if such Certificate shall be properly endorsed or otherwise be in proper form for transfer and the Person requesting such issuance shall pay any transfer or other taxes required by reason of the issuance of shares of IDEC Common Stock to a Person other than the registered holder of such Certificate or establish to the satisfaction of IDEC that such tax has been paid or is not applicable. Until surrendered as contemplated by this Section 2.2(b), each Certificate shall be deemed at any time after the Effective Time to represent only the right to receive the Merger Consideration (and any amounts to be paid pursuant to Section 2.1(e) or Section 2.2(c) ) upon such surrender. No interest shall be paid or shall accrue on any amount payable pursuant to Section 2.1(e) or Section 2.2(c).

        (c)    Distributions with Respect to Unexchanged Shares.    No dividends or other distributions with respect to IDEC Common Stock with a record date after the Effective Time shall be paid to the holder of any unsurrendered Certificate with respect to the shares of IDEC Common Stock represented thereby, and no cash payment in lieu of fractional shares shall be paid to any such holder pursuant to Section 2.1(e) hereof, until such Certificate has been surrendered in accordance with this Article II. Subject to Applicable Law (as defined in Section 3.1(g)(i)), following surrender of any such Certificate, there shall be paid to the recordholder thereof, without interest, (i) promptly after such surrender, the number of whole shares of IDEC Common Stock issuable in exchange therefor pursuant to this Article II, together with any cash payable in lieu of a fractional share of IDEC Common Stock to which such holder is entitled pursuant to Section 2.1(e) and the amount of dividends or other distributions with a record date after the Effective Time theretofore paid with respect to such whole shares of IDEC Common Stock, and (ii) at the appropriate payment date, the amount of dividends or other distributions with a record date after the Effective Time and a payment date subsequent to such surrender payable with respect to such whole shares of IDEC Common Stock.

        (d)    No Further Ownership Rights in Biogen Common Stock.    All shares of IDEC Common Stock issued upon the surrender for exchange of Certificates in accordance with the terms of this Article II and any cash paid pursuant to Section 2.1(e) or Section 2.2(c) shall be deemed to have been issued (and paid) in full satisfaction of all rights pertaining to the shares of Biogen Common Stock previously represented by such Certificates. At the Effective Time, the stock transfer books of Biogen shall be closed and there shall be no further registration of transfers on the stock transfer books of the Surviving Corporation of the shares of Biogen Common Stock which were outstanding immediately prior to the Effective Time. If, after the Effective Time, Certificates are presented to the Surviving Corporation or the Exchange Agent for any reason, they shall be canceled and exchanged as provided in this Article II.

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        (e)    Termination of Exchange Fund.    Any portion of the Exchange Fund which remains undistributed to the holders of Certificates one year after the Effective Time shall be delivered to IDEC, upon demand, and any holders of Certificates who have not theretofore complied with this Article II shall thereafter look only to IDEC for payment of their claim for the Merger Consideration, any cash in lieu of fractional shares of IDEC Common Stock pursuant to Section 2.1(e) and any dividends or distributions pursuant to Section 2.2(c).

        (f)    No Liability.    None of IDEC, Merger Sub, Biogen or the Exchange Agent shall be liable to any Person in respect of any shares of IDEC Common Stock (or dividends or distributions with respect thereto) or cash from the Exchange Fund delivered to a public official pursuant to any applicable abandoned property, escheat or similar law. If any Certificate shall not have been surrendered prior to seven years after the Effective Time, or immediately prior to such earlier date on which any shares of IDEC Common Stock, any cash in lieu of fractional shares of IDEC Common Stock or any dividends or distributions with respect to IDEC Common Stock issuable in respect of such Certificate would otherwise escheat to or become the property of any Governmental Entity (as defined in Section 3.1(c)(v)), any such shares, cash, dividends or distributions in respect of such Certificate shall, to the extent permitted by Applicable Law, become the property of the Surviving Corporation, free and clear of all claims or interest of any Person previously entitled thereto.

        (g)    Investment of Exchange Fund.    The Exchange Agent shall invest any cash included in the Exchange Fund as directed by IDEC on a daily basis provided that no such investment or loss thereon shall affect the amounts payable to former stockholders of Biogen after the Effective Time pursuant to this Article II. Any interest and other income resulting from such investment shall become a part of the Exchange Fund, and any amounts in excess of the amounts payable pursuant to this Article II shall promptly be paid to IDEC.

        (h)    Withholding Rights.    IDEC or the Exchange Agent shall be entitled to deduct and withhold from any consideration payable pursuant to this Agreement to any Person who was a holder of Biogen Common Stock immediately prior to the Effective Time such amounts as IDEC or the Exchange Agent may be required to deduct and withhold with respect to the making of such payment under the Code, or any provision of federal, state, local or foreign tax law. To the extent that amounts are so withheld by IDEC or the Exchange Agent, such withheld amounts shall be treated for all purposes of this Agreement as having been paid to the Person to whom such consideration would otherwise have been paid.

        (i)    Lost, Stolen or Destroyed Certificates.    In the event any Certificates shall have been lost, stolen or destroyed, the Exchange Agent shall issue in exchange for such lost, stolen or destroyed Certificates, upon the making of an affidavit of that fact by the holder thereof, such shares of IDEC Common Stock as may be required pursuant to Section 2.1(a), cash for fractional shares pursuant to Section 2.1(e) and any dividends or distributions payable pursuant to Section 2.2(c); provided, however, that IDEC may, in its discretion and as a condition precedent to the issuance thereof, require the owner of such lost, stolen or destroyed Certificates to deliver an agreement of indemnification in form reasonably satisfactory to IDEC, or a bond in such sum as IDEC may reasonably direct as indemnity, against any claim that may be made against IDEC or the Exchange Agent in respect of the Certificates alleged to have been lost, stolen or destroyed.

ARTICLE III

REPRESENTATIONS AND WARRANTIES

        SECTION 3.1    Representations and Warranties of Biogen.    Except as set forth in the disclosure schedule dated as of the date of this Agreement and executed and delivered by Biogen to IDEC concurrently with or prior to the execution and delivery by Biogen of this Agreement (the "Biogen

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Disclosure Schedule"), Biogen represents and warrants to IDEC and Merger Sub as set forth below. Each exception set forth in the Biogen Disclosure Schedule, and any other information included in the Biogen Disclosure Schedule, is identified by reference to, or has been grouped under a heading referring to, a specific individual subsection of this Agreement and shall be deemed to be disclosed solely for purposes of such subsection, except to the extent that disclosure in one subsection of the Biogen Disclosure Schedule is specifically referred to in another subsection of the Biogen Disclosure Schedule by appropriate cross-reference.

        (a)    Organization, Standing and Corporate Power; Charter Documents; Subsidiaries.    

        (i)    Organization, Standing and Corporate Power.    Biogen and each of its Subsidiaries (as defined in Section 8.3(p)) is a corporation or other legal entity duly organized, validly existing and in good standing (with respect to jurisdictions which recognize such concept) under the laws of the jurisdiction in which it is incorporated or otherwise organized, and has the requisite corporate (or similar) power and authority to carry on its business as currently conducted, except for those jurisdictions in which the failure to be so organized, existing or in good standing would not, individually or in the aggregate, reasonably be likely to have a Material Adverse Effect (as defined in Section 8.3(l)) on Biogen. Each of Biogen and its Subsidiaries is duly qualified or licensed to do business and is in good standing (with respect to jurisdictions which recognize such concept) in each jurisdiction in which the nature of its business or the ownership, leasing or operation of its properties makes such qualification or licensing necessary, except for those jurisdictions where the failure to be so qualified or licensed or to be in good standing would not, individually or in the aggregate, reasonably be likely to have a Material Adverse Effect on Biogen.

        (ii)    Charter Documents.    Biogen has delivered or made available to IDEC prior to the execution of this Agreement complete and correct copies of (A) the Articles of Organization of Biogen (including any certificates of designation), as amended and currently in effect (the "Biogen Charter"), and the by-laws of Biogen, as amended and currently in effect (the "Biogen By-Laws," and, together with the Biogen Charter, the "Biogen Organizational Documents"), and (B) the certificate of incorporation and by-laws or like organizational documents of each of the Biogen Significant Subsidiaries (as defined in Section 3.1(a)(iii)), as amended and currently in effect (collectively, the "Biogen Subsidiary Organizational Documents"), and each such instrument is in full force and effect. Biogen is not in violation of the Biogen Organizational Documents and no Biogen Significant Subsidiary is in violation of its Biogen Subsidiary Organizational Documents, except, in the case of such Biogen Significant Subsidiary, as would not reasonably be likely to have a Material Adverse Effect on Biogen.

        (iii)    Subsidiaries.    Section 3.1(a)(iii) of the Biogen Disclosure Schedule lists all the Subsidiaries of Biogen which as of the date of this Agreement are significant subsidiaries (as defined in Rule 102 of Regulation S-X of the U.S. Securities and Exchange Commission (the "SEC")) (the "Biogen Significant Subsidiaries"). All the outstanding shares of capital stock of, or other equity interests in, each Biogen Significant Subsidiary have been validly issued and are fully paid and nonassessable and are owned directly or indirectly by Biogen, free and clear of all pledges, claims, liens, charges, encumbrances and security interests of any kind or nature whatsoever (collectively, "Liens") and free of any other restriction (including any restriction on the right to vote, sell or otherwise dispose of such capital stock or other ownership interests).

        (b)    Capital Structure.    

        (i)    The authorized capital stock of Biogen consists of 375,000,000 shares of Biogen Common Stock, and 20,000,000 shares of preferred stock, par value $0.01 per share ("Biogen Preferred Stock"). At the close of business on May 31, 2003, (A) 148,964,583 shares of Biogen Common Stock were issued and outstanding; (B) 2,234,719 shares of Biogen Common Stock

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were held by Biogen in its treasury; (C) no shares of Biogen Preferred Stock were issued and outstanding; (D) 192,220 shares of Biogen Common Stock were reserved for issuance pursuant to the Biogen Purchase Plans; (E) 27,716,417 shares of Biogen Common Stock were reserved for issuance pursuant to the Biogen 1982 Incentive Stock Option Plan, as amended and restated through December 13, 2002, effective December 31, 2002, the Biogen 1985 Non-Qualified Stock Option Plan, as amended and restated through April 11, 2003, the Biogen 1987 Scientific Board Stock Option Plan, as amended and restated through February 7, 2003, and the Biogen 2003 Omnibus Equity Plan (such plans, collectively, the "Biogen Stock Plans"), complete and correct copies of which, in each case as amended, have been filed as exhibits to the Biogen SEC Documents (as defined in Section 3.1(d)(i)) prior to the date of this Agreement or delivered to IDEC; and (F) 250,000 shares of Biogen Preferred Stock were designated as Series A-1 Junior Participating Preferred Stock, par value $0.01 per share, and were reserved for issuance upon the exercise of preferred share purchase rights (the "Biogen Rights") issued pursuant to the Biogen Rights Agreement dated May 8, 1999 between Biogen and State Street Bank and Trust Company (as successor in interest to First National Bank of Boston), as amended by the Amendment and Agreement to Rights Agreement dated as of May 31, 2002 between Biogen, State Street Bank and Trust Company, as the old rights agent, and EquiServe Trust Company, N.A., as the new rights agent, and the Second Agreement and Amendment to Rights Agreement dated as of June 20, 2003, between Biogen and EquiServe Trust Company, N.A. (the "Biogen Rights Agreement").

        (ii)   As of the close of business on May 31, 2003, 18,852,124 shares of Biogen Common Stock were subject to issuance pursuant to outstanding Biogen Options under the Biogen Stock Plans. All shares of Biogen Common Stock subject to issuance under the Biogen Stock Plans and the Biogen Purchase Plans, upon issuance on the terms and conditions specified in the instruments pursuant to which they are issuable, will be duly authorized, validly issued, fully paid and nonassessable. There are no commitments or agreements of any character to which Biogen is bound obligating Biogen to accelerate the vesting of any Biogen Option as a result of the Merger (whether alone or upon the occurrence of any additional or subsequent events). There are no outstanding or authorized stock appreciation, phantom stock, profit participation or other similar rights with respect to Biogen.

        (iii)  No bonds, debentures, notes or other evidences of indebtedness having the right to vote on any matters on which stockholders may vote ("Voting Debt") of Biogen are issued or outstanding as of the date hereof.

        (iv)  Except as otherwise set forth in this Section 3.1(b), as of May 31, 2003, there are no securities, options, warrants, calls, rights, commitments, agreements, arrangements or undertakings of any kind to which Biogen or any of its Subsidiaries is a party or by which any of them is bound obligating Biogen or any of its Subsidiaries to issue, deliver or sell, or cause to be issued, delivered or sold, additional shares of capital stock, Voting Debt or other voting securities of Biogen or any of its Subsidiaries, or obligating Biogen or any of its Subsidiaries to issue, grant, extend or enter into any such security, option, warrant, call, right, commitment, agreement, arrangement or undertaking. All outstanding shares of Biogen Common Stock, all outstanding Biogen Options, and all outstanding shares of capital stock of each Subsidiary of Biogen have been issued and granted in compliance in all material respects with (A) all applicable securities laws and all other Applicable Law and (B) all requirements set forth in applicable material Contracts (as defined in Section 8.3(d)).

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        (v)   Since May 31, 2003 and through the date hereof, other than (A) issuances of Biogen Common Stock pursuant to the exercise of Biogen Options granted under Biogen Stock Plans and outstanding as of May 31, 2003, (B) issuances of Biogen Common Stock pursuant to the Biogen Purchase Plans, (C) repurchases of Biogen Common Stock from employees of Biogen following their termination pursuant to the terms of their pre-existing stock option or purchase agreements, (D) issuances of Biogen Common Stock (consisting of newly-issued shares or shares in treasury) as contributions of Biogen Common Stock to defined contribution plans sponsored by Biogen ("Biogen Defined Contribution Plans") and (E) grants of Biogen Options under Biogen Stock Plans in the ordinary course of business consistent with past practice, there has been no change in (1) the outstanding capital stock of Biogen, (2) the number of Biogen Options outstanding, or (3) the number of other options, warrants or other rights to purchase Biogen capital stock.

        (vi)  Neither Biogen nor any Subsidiary of Biogen is a party to any currently effective agreement restricting the purchase or transfer of, relating to the voting of, requiring registration of, or granting any preemptive or antidilutive rights with respect to, any capital stock of Biogen or any of its Subsidiaries or any securities of the type referred to in Section 3.1(b)(iv) hereof.

        (vii) Other than its Subsidiaries, as of the date hereof, Biogen does not directly or indirectly beneficially own any securities or other beneficial ownership interests in any other entity except for non-controlling investments made in the ordinary course of business consistent with past practice in entities which are not individually or in the aggregate material to Biogen and its Subsidiaries taken as a whole.

        (c)    Authority; Board Approval; Voting Requirements; No Conflict; Required Filings and Consents.    

        (i)    Authority.    Biogen has all requisite corporate power and authority to enter into this Agreement and to consummate the transactions contemplated hereby. The execution and delivery of this Agreement by Biogen, and the consummation by Biogen of the transactions contemplated hereby, have been duly authorized by all necessary corporate action on the part of Biogen, and no other corporate proceedings on the part of Biogen and no stockholder votes are necessary to authorize this Agreement or to consummate the transactions contemplated hereby, other than with respect to approval of this Agreement and the Merger, the Biogen Stockholder Approval. This Agreement has been duly executed and delivered by Biogen. Assuming the due authorization, execution and delivery of this Agreement by IDEC and Merger Sub, this Agreement constitutes the legal, valid and binding obligation of Biogen enforceable against Biogen in accordance with its terms, subject to applicable bankruptcy, insolvency, reorganization, moratorium or other laws relating to or affecting the rights and remedies of creditors generally and to general principles of equity (regardless of whether considered in a proceeding in equity or at law).

        (ii)    Board Approval.    The Board of Directors of Biogen has (A) determined that this Agreement and the Merger are advisable and fair to and in the best interest of Biogen and its stockholders, (B) approved and adopted this Agreement, the Merger and the other transactions contemplated hereby, which adoption has not been rescinded or modified, (C) resolved (subject to Section 4.2(d)) to recommend this Agreement and the Merger to its stockholders for approval and (D) directed that this Agreement and the Merger be submitted to its stockholders for consideration in accordance with this Agreement.

        (iii)    Voting Requirements.    The affirmative vote in favor of approval of this Agreement and the Merger by the holders of a majority of the outstanding shares of Biogen Common Stock entitled to vote thereon at a duly convened and held Biogen Stockholders' Meeting (the

9

"Biogen Stockholder Approval") is the only vote of the holders of any class or series of Biogen's capital stock necessary to approve and adopt this Agreement, the Merger and the other transactions contemplated hereby.

        (iv)    No Conflict.    The execution and delivery of this Agreement by Biogen does not, and the consummation by Biogen of the transactions contemplated hereby and compliance by Biogen with the provisions of this Agreement will not, conflict with, or result in any violation of, or default (with or without notice or lapse of time, or both) under, require any consent, waiver or approval under, give rise to any right of termination or other right, or the cancellation or acceleration of any right or obligation or loss of a benefit under, or result in the creation of any Lien upon any of the properties or assets of Biogen or any of its Subsidiaries or any restriction on the conduct of Biogen's business or operations under, (A) the Biogen Organizational Documents or the Biogen Subsidiary Organizational Documents, (B) any Contract, permit, concession, franchise, license or authorization applicable to Biogen or any of its Subsidiaries or their respective properties or assets, or (C) subject to the governmental filings and other matters referred to in Section 3.1(c)(v), any judgment, order, decree, statute, law, ordinance, rule or regulation applicable to Biogen or any of its Subsidiaries or their respective properties or assets, other than, in the case of clauses (B) and (C), any such conflicts, violations, defaults, rights, losses, restrictions or Liens, or failure to obtain consents, waivers or approvals, which, individually or in the aggregate, would not reasonably be likely to have a Material Adverse Effect on Biogen.

        (v)    Required Filings or Consents.    No consent, approval, order or authorization of, action by or in respect of, or registration, declaration or filing with, any federal, state, local or foreign government, any court, administrative, regulatory or other governmental agency, commission or authority or any non-governmental self-regulatory agency, commission or authority (a "Governmental Entity") is required to be made or obtained by or with respect to Biogen or any of its Subsidiaries in connection with the execution and delivery of this Agreement by Biogen or the consummation by Biogen of the transactions contemplated hereby, except for:

        (A)  the filing of a pre-merger notification and report form by Biogen under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended (the "HSR Act"), and any applicable filings or notifications under the antitrust, competition or similar laws of any foreign jurisdiction;

        (B)  the filing with the SEC of:

        (1)   a proxy statement relating to the Biogen Stockholders' Meeting (such proxy statement, together with the proxy statement relating to the IDEC Stockholders' Meeting (as defined in Section 5.1(b)), in each case as amended or supplemented from time to time, the "Joint Proxy Statement");

        (2)   such reports under Section 13(a), 13(d), 15(d) or 16(a) of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), as may be required in connection with this Agreement and the transactions contemplated hereby;

        (C)  the filing of the Articles of Merger with the Secretary of State and appropriate documents with the relevant authorities of other states in which Biogen is qualified to do business and such filings with state securities or other "blue sky" laws; and

        (D)  such consents, approvals, orders or authorizations, the failure of which to be made or obtained, individually or in the aggregate, would not reasonably be likely to have a Material Adverse Effect on Biogen.

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        (d)    SEC Documents; Financial Statements.    

        (i)    Biogen has filed with the SEC all registration statements, prospectuses, reports, schedules, forms, statements, certifications and other documents (including exhibits and all other information incorporated by reference therein) required to be so filed by Biogen since January 1, 2001 (excluding the Joint Proxy Statement, the "Biogen SEC Documents"). As of their respective dates, the Biogen SEC Documents complied in all material respects with the requirements of the Securities Act of 1933, as amended (the "Securities Act"), or the Exchange Act, as the case may be, and the rules and regulations of the SEC promulgated thereunder applicable to such Biogen SEC Documents, and none of the Biogen SEC Documents, when filed, contained any untrue statement of a material fact or omitted to state a material fact required to be stated therein or necessary in order to make the statements therein, in light of the circumstances under which they were made, not misleading, except to the extent corrected by a subsequently filed Biogen SEC Document filed with the SEC prior to the date hereof.

        (ii)   The financial statements of Biogen included in the Biogen SEC Documents, including each Biogen SEC Document filed after the date hereof until the Effective Time, comply, as of their respective dates of filing with the SEC, in all material respects with applicable accounting requirements and the published rules and regulations of the SEC with respect thereto, have been prepared in accordance with United States generally accepted accounting principles ("GAAP") (except, in the case of unaudited statements, as permitted by Form 10-Q or 8-K of the SEC) applied on a consistent basis during the periods involved (except as may be indicated in the notes thereto) and fairly present the consolidated financial position of Biogen and its consolidated Subsidiaries as of the dates thereof and the consolidated results of their operations and cash flows for the periods then ended (subject, in the case of unaudited statements, to normal year-end audit adjustments which are not material).

        (iii)  Except as reflected or reserved against in the balance sheet of Biogen dated March 31, 2003 included in the Form 10-Q filed by Biogen with the SEC on April 17, 2003 (including the notes thereto, the "Biogen Balance Sheet"), neither Biogen nor any of its Subsidiaries has any liabilities (absolute, accrued, contingent or otherwise) which are required by GAAP to be set forth on a consolidated balance sheet of Biogen and its consolidated Subsidiaries or in the notes thereto, other than (A) liabilities and obligations incurred since March 31, 2003 in the ordinary course of business consistent with past practice which would not reasonably be likely to have a Material Adverse Effect on Biogen and (B) liabilities and obligations incurred in connection with this Agreement or the transactions contemplated hereby.

        (iv)  Neither Biogen nor any of its Subsidiaries is a party to, or has any commitment to become a party to, any joint venture, partnership or any similar contract or arrangement (including without limitation any contract or arrangement relating to any transaction or relationship between or among Biogen and any of its Subsidiaries, on the one hand, and any unconsolidated Affiliate (as defined in Section 8.3(a)), including without limitation any structured finance, special purpose or limited purpose entity or Person, on the other hand), where the result, purpose or intended effect of such contract or arrangement is to avoid disclosure of any material transaction involving, or material liabilities of, Biogen or any of its Subsidiaries in Biogen's or its Subsidiaries' published financial statements.

        (e)    Information Supplied.    None of the information supplied or to be supplied by or on behalf of Biogen for inclusion or incorporation by reference in (i) the registration statement on Form S-4 to be filed with the SEC by IDEC in connection with the issuance of IDEC Common Stock in the Merger (including any amendments or supplements, the "Form S-4") will, at the time

11

the Form S-4 becomes effective under the Securities Act, contain any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary to make the statements therein not misleading or (ii) the Joint Proxy Statement will, at the date it is first mailed to Biogen's stockholders or at the time of the Biogen Stockholders' Meeting, contain any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary in order to make the statements therein, in light of the circumstances under which they are made, not misleading. The Joint Proxy Statement and the Form S-4 will comply as to form in all material respects with the requirements of the Securities Act and the Exchange Act and the rules and regulations thereunder, except that no representation or warranty is made by Biogen with respect to information or statements with respect to IDEC or its Subsidiaries made or incorporated by reference therein supplied by or on behalf of IDEC for inclusion or incorporation by reference in the Joint Proxy Statement or the Form S-4.

        (f)    Absence of Certain Changes or Events.    

        (i)    Since December 31, 2002 through the date hereof, except as and to the extent disclosed in the Biogen SEC Documents filed prior to the date of this Agreement:

        (A)  Biogen and its Subsidiaries have conducted their business only in the ordinary course consistent with past practice;

        (B)  there has not been any split, combination or reclassification of any of Biogen's capital stock or any declaration, setting aside or payment of any dividend on, or other distribution (whether in cash, stock or property) in respect of, in lieu of, or in substitution for, shares of Biogen's capital stock;

        (C)  except as required by a change in GAAP, there has not been any change in accounting methods, principles or practices by Biogen; and

        (D)  there has not been any action taken by Biogen or any of its Subsidiaries that, if taken during the period from the date of this Agreement through the Effective Time, would constitute a breach of Section 4.1(b), other than actions in connection with entering into this Agreement.

        (ii)   Since December 31, 2002 through the date hereof, there have not been any changes, circumstances or events that, individually or in the aggregate, have had, or would reasonably be likely to have, a Material Adverse Effect on Biogen.

        (g)    Compliance with Applicable Laws; Permits; Litigation.    

        (i)    Biogen, its Subsidiaries and employees hold all permits, licenses, easements, variances, exemptions, orders, consents, registrations and approvals of all Governmental Entities (including all authorizations under the Federal Food, Drug and Cosmetic Act of 1938, as amended (the "FDCA"), and the regulations of the U.S. Food and Drug Administration (the "FDA") promulgated thereunder) which are required for the operation of the businesses of Biogen and its Subsidiaries in the manner described in the Biogen SEC Documents filed prior to the date hereof or as they are being conducted as of the date hereof (the "Biogen Permits"), and all Biogen Permits are in full force and effect, except where the failure to have, or the suspension or cancellation of, or the failure to be valid or in full force and effect of, any such Biogen Permits individually or in the aggregate would not reasonably be likely to have a Material Adverse Effect on Biogen. As of the date hereof, Biogen and its Subsidiaries are in compliance with the terms of the Biogen Permits and all applicable laws, statutes, orders, rules, regulations, policies or guidelines promulgated, or judgments, decisions or orders entered by any Governmental Entity (all such laws, statutes, orders, rules, regulations, policies, guidelines, judgments, decisions and orders, collectively, "Applicable Laws") relating to Biogen

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and its Subsidiaries or their respective business or properties, except where the failure to be in compliance with the terms of the Biogen Permits or such Applicable Laws individually or in the aggregate would not reasonably be likely to have a Material Adverse Effect on Biogen.

        (ii)   As of the date hereof, except as and to the extent disclosed in the Biogen SEC Documents filed prior to the date of this Agreement, including the notes to the financial statements included therein, no action, demand, suit, proceeding, requirement or investigation by any Governmental Entity and no suit, action, mediation, arbitration or proceeding by any Person, against or affecting Biogen or any of its Subsidiaries or any of their respective properties, including Intellectual Property (as defined in Section 8.3(j)), is pending or, to the Knowledge (as defined in Section 8.3(k)) of Biogen, threatened which individually or in the aggregate would reasonably be likely to have a Material Adverse Effect on Biogen.

        (iii)  As of the date hereof, neither Biogen nor any of its Subsidiaries is subject to any outstanding order, injunction or decree which, individually or in the aggregate, has had or would reasonably be likely to have a Material Adverse Effect on Biogen.

        (h)    Labor and Other Employment Matters.    

        (i)    As of the date hereof, except as would not, individually or in the aggregate, reasonably be likely to have a Material Adverse Effect on Biogen, (a) no work stoppage, slowdown, lockout, labor strike, material arbitrations or other material labor disputes against Biogen or any of its Subsidiaries are pending or, to the Knowledge of Biogen, threatened, (b) no unfair labor practice charges, grievances or complaints are pending or, to the Knowledge of Biogen, threatened against Biogen or any of its Subsidiaries, (c) neither Biogen nor any of its Subsidiaries is delinquent in payments to any of its employees for any wages, salaries, commissions, bonuses or other direct compensation for any services performed for it or amounts required to be reimbursed to such employees, (d) Biogen and each of its Subsidiaries are in compliance with all Applicable Laws respecting labor and employment, including, but not limited to, terms and conditions of employment, workers' compensation, occupational safety and health requirements, plant closings, wages and hours, employment discrimination, disability rights or benefits, equal opportunity, affirmative action, employee benefits, severance payments, labor relations, employee leave issues and unemployment insurance and related matters, (e) there are no complaints, charges or claims against Biogen or any of its Subsidiaries pending with or, to the Knowledge of Biogen, threatened by any Governmental Entity or arbitrator based on, arising out of, in connection with, or otherwise relating to the employment of any employees by Biogen and or any of its Subsidiaries, other than those occurring in the ordinary course of business, such as claims for workers' compensation or unemployment benefits, (f) Biogen and each of its Subsidiaries have withheld all amounts required by Applicable Law to be withheld from the wages, salaries, and other payments to employees, and is not liable for any arrears of wages or any Taxes (as defined in Section 3.1(j)(xi)) or any penalty for failure to comply with any of the foregoing, (g) neither Biogen nor any of its Subsidiaries is liable for any payment to any trust or other fund or to any Governmental Entity, with respect to unemployment compensation benefits, social security or other benefits or obligations for employees (other than routine payments to be made in the ordinary course of business consistent with past practice), (h) no employee of Biogen or any of its Subsidiaries, at the officer level or above, has given notice to Biogen or any of its Subsidiaries that any such employee intends to terminate his or her employment with Biogen or any of its Subsidiaries, and (i) to the Knowledge of Biogen, no employee of Biogen or any of its Subsidiaries is in any respect in violation of any term of any employment contract, non-disclosure agreement, noncompetition agreement, or any restrictive covenant to a former employer relating to the right of any such employee to be employed by Biogen or any of its Subsidiaries because of the nature of the business conducted or presently proposed to be

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conducted by Biogen or any of its Subsidiaries or to the use of trade secrets or proprietary information of others.

        (ii)   As of the date hereof,

        (A)  neither Biogen nor any of its Subsidiaries is a party to, or otherwise bound by, any collective bargaining agreement or any other agreement with a labor union or labor organization, nor is any such agreement presently being negotiated;

        (B)  no labor organization or group of employees of Biogen or any of its Subsidiaries has made a pending demand for recognition or certification, and there are no representation or certification proceedings or petitions seeking a representation proceeding presently pending or, to the Knowledge of Biogen, threatened to be brought or filed with the National Labor Relations Board or any other labor relations tribunal or authority; and

        (C)  to the Knowledge of Biogen, no labor union is seeking to organize any employees of Biogen or any of its Subsidiaries.

        (i)    Benefit Plans.    

        (i)    With respect to each bonus, pension, profit sharing, deferred compensation, incentive compensation, stock ownership, stock purchase, stock option, phantom stock, stock-related or performance award, retirement, vacation, severance, disability, death benefit, hospitalization, medical, loan, fringe benefit, disability, sabbatical and other similar plan, arrangement or understanding, including, without limitation, each "employee benefit plan" within the meaning of Section 3(3) of the Employee Retirement Income Security Act of 1974, as amended ("ERISA") and any employment agreement, consulting agreement or severance agreement (such plans, agreements, arrangements or understandings, collectively, "Benefit Plans") with or for the benefit of any current or former employee, officer or director of Biogen or any of its Subsidiaries or ERISA Affiliates (as defined in Section 3.1(i)(vi)) (the "Biogen Benefit Plans"), no event has occurred and there exists no condition or set of circumstances, which could reasonably be likely to have a Material Adverse Effect on Biogen under ERISA, the Code or any other Applicable Law.

        (ii)   Each Biogen Benefit Plan has been, in all material respects, administered and operated in accordance with its terms, with the applicable provisions of ERISA, the Code and other Applicable Law and the terms of all applicable collective bargaining agreements. Each Biogen Benefit Plan, including any material amendments thereto, that is capable of approval by, or registration or qualification for special tax status with, the appropriate taxation, social security or supervisory authorities in the relevant country, state, territory or the like (each, an "Approval") has received such Approval (or there remains a period of time in which to obtain such Approval retroactive to the date of any material amendment that has not previously received such Approval) and no event has occurred which would be reasonably likely to result in the revocation of such Approval or the imposition of material sanctions by such authorities.

        (iii)  To the Knowledge of Biogen, no oral or written representation or commitment with respect to any material aspect of any Biogen Benefit Plan has been made to an employee or former employee of Biogen or any of its Subsidiaries by an authorized Biogen employee that is not materially in accordance with the written or otherwise preexisting terms and provisions of such Biogen Benefit Plans. To the Knowledge of Biogen, neither Biogen nor any of its Subsidiaries has entered into any agreement, arrangement or understanding, whether written or oral, with any trade union, works council or other employee representative body or any material number or category of its employees which would prevent, restrict or materially

14

impede the implementation of any lay-off, redundancy, severance or similar program within its or their respective workforces (or any part of them).

        (iv)  There are no material unresolved claims or disputes under the terms of, or in connection with, any Biogen Benefit Plan (other than routine undisputed claims for benefits), and no action, legal or otherwise, has been commenced or threatened with respect to any material claim or otherwise in connection with a Biogen Benefit Plan.

        (v)   With respect to each Funded Retirement Plan (as defined below) of Biogen or its Subsidiaries, the aggregate value of the assets of such Funded Retirement Plan is equal to or greater than the aggregate value of its liabilities assessed on an ongoing and terminated basis and calculated in accordance with the actuarial methods and assumptions used in such valuation pursuant to such Funded Retirement Plan and Applicable Law and GAAP. For purposes of this Agreement, "Funded Retirement Plan" means, with respect to a party, a Benefit Plan that is a "pension plan" within the meaning of Section 3(2) of ERISA (whether or not such Benefit Plan is subject to ERISA) and under which the assets to satisfy the benefit obligations are legally segregated from the general assets of such party or its Subsidiaries and are not subject to the creditors of such party or its Subsidiaries. None of Biogen or any other person or entity under common control within the meaning of Section 414(b), (c), (m) or (o) of the Code (an "ERISA Affiliate") with Biogen has incurred any liability to a Funded Retirement Plan under Title IV of ERISA (other than for contributions not yet due) or to the Pension Benefit Guaranty Corporation (other than for payment of premiums not yet due) that, when aggregated with other such liabilities, would result in a material liability of Biogen and its Subsidiaries taken as a whole, which liability has not been fully paid.

        (vi)  At no time has Biogen or any ERISA Affiliate of Biogen participated in and/or been obligated to contribute to any Benefit Plan that is a "multiemployer plan" within the meaning of Section 3(37) of ERISA.

        (vii) No Biogen Benefit Plan provides health benefits (whether or not insured), with respect to employees or former employees of Biogen or any Subsidiary of Biogen after retirement or other termination of service (other than coverage mandated by Applicable Law or benefits, the full cost of which is borne by the employee or former employee).

        (viii) Neither the negotiation and execution of this Agreement nor the consummation of the transactions contemplated hereby will (either alone or upon the occurrence of any additional or subsequent events) constitute an event under any Biogen Benefit Plan that will or may result in any payment (whether of severance pay or otherwise), acceleration of payment, forgiveness of indebtedness, vesting, distribution, increase in benefits or obligation to fund benefits with respect to any employee or former employee of Biogen or any Subsidiary of Biogen. There is no contract, agreement, plan or arrangement with an employee or former employee of Biogen to which Biogen or any of its Subsidiaries is a party as of the date of this Agreement, that, individually or collectively and as a result of the transaction contemplated hereby (whether alone or upon the occurrence of any additional or subsequent events) or otherwise, would reasonably be likely to give rise to the payment of any amount that would not be deductible pursuant to Sections 280G or 162(m) of the Code.

        (j)    Taxes.    

        (i)    Each of Biogen and its Subsidiaries, including any predecessors thereof, has (A) duly and timely filed (or there have been filed on its behalf) all material Tax Returns (as defined below) required to be filed by it (taking into account all applicable extensions) with the appropriate Tax Authority (as defined below) and all such Tax Returns are true, correct and complete in all material respects, (B) duly paid in full or made provision in accordance with

15

GAAP (or there has been paid or provision has been made on its behalf) for the payment of all material Taxes for all periods ending through the date hereof, and (C) complied in all material respects with all Applicable Laws relating to the payment and withholding of Taxes.

        (ii)   There are no material Liens for Taxes upon any property or assets of Biogen or any of its Subsidiaries, except for liens for Taxes not yet due and payable and for which adequate reserves have been provided in accordance with GAAP in the most recent financial statements contained in the Biogen SEC Documents filed prior to the date of this Agreement.

        (iii)  The unpaid Taxes of Biogen and its Subsidiaries (A) did not, as of the most recent financial statements contained in the Biogen SEC Documents filed prior to the date of this Agreement, exceed the reserve for Tax liability (rather than any reserve for deferred Taxes established to reflect timing differences between book and Tax income) set forth on the face of such financial statements (rather than in any notes thereto), and (B) do not exceed that reserve as adjusted for the passage of time since the date of such financial statements in accordance with the past custom and practice of Biogen and its Subsidiaries in filing their Tax Returns.

        (iv)  There is no audit, examination, deficiency, refund litigation, proposed adjustment or matter in controversy with respect to any Taxes or Tax Return of Biogen or its Subsidiaries which if determined adversely would be expected to result in a material Tax deficiency. Neither Biogen nor any of its Subsidiaries has received notice of any claim made by a governmental authority in a jurisdiction where Biogen or any of its Subsidiaries, as applicable, does not file a Tax Return, that Biogen or such Subsidiary is or may be subject to taxation by that jurisdiction.

        (v)   The material income Tax Returns of Biogen and each of its Subsidiaries, including any predecessors thereof, have been examined by the applicable Tax Authority (or the applicable statutes of limitations for the assessment of income Taxes for such periods have expired) for all periods through and including December 31, 1997, and no material deficiencies were asserted as a result of such examinations which have not been resolved and fully paid or accrued as a liability on the most recent financial statements contained in the Biogen SEC Documents.

        (vi)  There are no outstanding requests, agreements, consents or waivers to extend the statutory period of limitations applicable to the assessment of any Taxes or deficiencies against Biogen or any of its Subsidiaries, and no power of attorney granted by either Biogen or any of its Subsidiaries with respect to any Taxes is currently in force.

        (vii) Neither Biogen nor any of its Subsidiaries is a party to any agreement providing for the allocation or sharing of Taxes imposed on or with respect to any individual or other Person, and neither Biogen nor any of its Subsidiaries (A) has been a member of an affiliated group (or similar state, local or foreign filing group) filing a consolidated income Tax Return (other than the group the common parent of which is Biogen) or (B) has any liability for the Taxes of any Person (other than Biogen or any of its Subsidiaries) under Treasury Regulation § 1.1502-6 (or any similar provision of state, local or foreign law), as a transferee or successor, by contract, or otherwise.

        (viii) Biogen and each of its Subsidiaries has delivered or made available to IDEC complete copies of all material income Tax Returns of Biogen and each of its Subsidiaries, including any predecessors thereof, for taxable years ending between January 1, 1999 and December 31, 2002, excepting such Tax Returns as have not been filed for the taxable year ending December 31, 2002 pursuant to appropriate extensions with respect thereto.

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        (ix)  Neither Biogen nor any of its Subsidiaries has: (A) filed a consent under Section 341(f) of the Code concerning collapsible corporations; (B) agreed to make nor is it required to make any material adjustment under Section 481(a) of the Code by reason of a change in accounting method or otherwise; (C) constituted either a "distributing corporation" or a "controlled corporation" (within the meaning of Section 355(a)(1)(A) of the Code) in a distribution of stock qualifying for tax-free treatment under Section 355 of the Code (I) in the two years prior to the date of this Agreement or (II) in a distribution which could otherwise constitute part of a "plan" or "series of related transactions" (within the meaning of Section 355(e) of the Code) in connection with the Merger; or (D) taken any action or knows of any fact, agreement, plan or other circumstance that is reasonably likely to prevent the Merger from qualifying as a "reorganization" within the meaning of Section 368(a) of the Code.

        (x)   Neither Biogen nor any of its Subsidiaries is, or has been, a United States real property holding corporation (as defined in Section 897(c)(2) of the Code) during the applicable period specified in Section 897(c)(1)(A)(ii) of the Code.

        (xi)  For purposes of this Agreement, "Audit" means any examination, audit, assessment, action, proceeding, investigation, dispute or claim with respect to Taxes by any Tax Authority or any administrative or judicial proceedings or appeals of such proceedings relating to Taxes; "Taxes" means any and all federal, state, local, foreign or other taxes of any kind (together with any and all interest, penalties, additions to tax and additional amounts imposed with respect thereto) imposed by any governmental authority, including, without limitation, taxes or other charges on or with respect to income, franchises, windfall or other profits, gross receipts, property, sales, use, capital stock, payroll, employment, unemployment, social security, workers' compensation, or net worth, and taxes or other charges in the nature of excise, withholding, ad valorem or value added; "Tax Authority" means the Internal Revenue Service and any other domestic or foreign governmental authority responsible for the administration or collection of any Taxes; and "Tax Return" means any return, report or similar statement (including the attached schedules) required to be filed with respect to Taxes, including, without limitation, any information return, claim for refund, amended return, or declaration of estimated Taxes.

        (k)    Regulatory Compliance; Supply.    

        (i)    All biological and drug products currently being manufactured, distributed or developed by Biogen or its Subsidiaries, or by any other Person pursuant to a development, commercialization, manufacturing, supply or other collaboration arrangement (other than an arrangement that is solely a license to market, distribute or sell products in a specified territory) with Biogen or any of its Subsidiaries (any such other Person acting under or in respect of such an arrangement, a "Biogen Collaboration Partner" and any such an arrangement with Biogen or any of its Subsidiaries, a "Biogen Collaboration"), that are subject to the jurisdiction of the FDA are, or, in the case of such manufacture, distribution or development by a Biogen Collaboration Partner pursuant to a Biogen Collaboration, to the Knowledge of Biogen are, being manufactured, labeled, stored, tested, distributed, and marketed in compliance with all applicable requirements under the FDCA, the Public Health Service Act, Biological Products, 21 C.F.R. §§ 600-610 (the "Public Health Service Act"), and their applicable implementing regulations, except for such noncompliance which would not, individually or in the aggregate, reasonably be likely to have a Material Adverse Effect on Biogen. The biological and drug products described in this Section 3.1(k)(i) are sometimes referred to herein as "Biogen Pharmaceutical Products."

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        (ii)   All clinical trials being conducted by Biogen or its Subsidiaries are, or, in the case of such trials being conducted by a Biogen Collaboration Partner pursuant to a Biogen Collaboration, to the Knowledge of Biogen are, being conducted in material compliance with the applicable requirements of Good Clinical Practice (as defined in Section 8.3(g)) and all applicable requirements relating to protection of human subjects contained in 21 C.F.R. Parts 50, 54, and 56, except for such noncompliance which would not, individually or in the aggregate, reasonably be likely to have a Material Adverse Effect on Biogen.

        (iii)  The manufacture of Biogen Pharmaceutical Products is, or, in the case of any Biogen Pharmaceutical Products manufactured by a Biogen Collaboration Partner pursuant to a Biogen Collaboration, to the Knowledge of Biogen is, being conducted in compliance with the FDA's applicable current Good Manufacturing Practice (as defined in Section 8.3(h)) regulations for drug and biological products, except for such noncompliance which would not, individually or in the aggregate, reasonably be likely to have a Material Adverse Effect on Biogen. In addition, Biogen and its Subsidiaries and, to the Knowledge of Biogen, their respective Biogen Collaboration Partners with respect to their activities pursuant to a Biogen Collaboration, are in compliance with all applicable registration and listing requirements set forth in 21 U.S.C. Section 360 and 21 C.F.R. Part 207 and all similar Applicable Laws, except for such noncompliance which would not, individually or in the aggregate, reasonably be likely to have a Material Adverse Effect on Biogen.

        (iv)  No Biogen Pharmaceutical Product has been recalled, suspended or discontinued as a result of any action by the FDA or similar foreign Governmental Entity.

        (v)   Since January 1, 2001, none of Biogen, its Subsidiaries nor, to the Knowledge of Biogen, their respective Collaboration Partners, has received any notice that the FDA or any other Governmental Entity has initiated or is considering initiating any steps, procedures or action to withdraw approval for or enjoin the production, sale, marketing or reimbursement of, or request the recall of, any Biogen Pharmaceutical Product.

        (vi)  None of Biogen, any of its Subsidiaries, nor, to the Knowledge of Biogen, any of their respective Biogen Collaboration Partners with respect to their activities pursuant to a Biogen Collaboration, has committed any act, made any statement or failed to make any statement that would reasonably be likely to provide a basis for the FDA to invoke its policy with respect to "Fraud, Untrue Statements of Material Facts, Bribery, and Illegal Gratuities" set forth in 56 Fed. Reg. 46191 (September 10, 1991) and any amendments thereto. Additionally, none of Biogen, its Subsidiaries nor, to the Knowledge of Biogen, any of their respective officers, key employees, agents, nor, to the Knowledge of Biogen, any Biogen Collaboration Partner nor any officer, key employee or agent thereof, has been convicted of any crime or engaged in any conduct (in the case of any Biogen Collaboration Partner or any officer, key employee or agent thereof, arising out of or in connection with acts or omissions pursuant to a Biogen Collaboration) that would reasonably be likely to result in (i) debarment under 21 U.S.C. Section 335a or any similar state law or regulation or (ii) exclusion under 42 U.S.C. Section 1320a-7 or any similar state law or regulation.

        (vii) Biogen has, or, to the Knowledge of Biogen, the Biogen Collaboration Partners that manufacture Biogen Pharmaceutical Products pursuant to a Biogen Collaboration have, sufficient manufacturing capacity and access to sufficient supply of materials to satisfy the reasonably anticipated clinical and commercial requirements for Biogen Pharmaceutical Products.

        (l)    Environmental Matters.    As of the date hereof, except as would not, individually or in the aggregate, reasonably be likely to have a Material Adverse Effect on Biogen, (i) the operations of Biogen and its Subsidiaries are in compliance with all applicable Environmental Laws (as defined

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in Section 8.3(e)), including possession and compliance with the terms of all licenses required by Environmental Laws, (ii) there are no pending, or to the Knowledge of Biogen, threatened suits, actions, investigations or proceedings under or pursuant to Environmental Laws against Biogen or its Subsidiaries or involving any real property currently or, to the Knowledge of Biogen, formerly owned, operated or leased by Biogen or its Subsidiaries, (iii) Biogen and its Subsidiaries are not subject to and have received no written allegations of any Environmental Liabilities (as defined in Section 8.3(f)) and, to the Knowledge of Biogen, no facts, circumstances or conditions relating to, arising from, associated with or attributable to any real property currently or, to the Knowledge of Biogen, formerly owned, operated or leased by Biogen or its Subsidiaries or operations thereon has resulted in or would reasonably be likely to result in Environmental Liabilities, and (iv) all real property owned or operated by Biogen or its Subsidiaries is free of contamination from Hazardous Materials (as defined in Section 8.3(i)) that would have an adverse effect on human health or the environment.

        (m)    Intellectual Property.    Except as would not, individually or in the aggregate, reasonably be expected to have a Material Adverse Effect on Biogen, (i) Biogen and each of its Subsidiaries owns or has a legally enforceable right to use (in each case, free and clear of any Liens) all Intellectual Property used in or necessary for the conduct of its business as currently conducted, including without limitation all patents and patent applications and all trademark registrations and trademark applications; (ii) to the Knowledge of Biogen, the conduct of the business of Biogen and its Subsidiaries as currently conducted does not infringe on or misappropriate, either directly or indirectly, the Intellectual Property rights of any Person and the use by Biogen or its Subsidiaries of any Intellectual Property is, to the Knowledge of Biogen, in accordance with any applicable grant, license, agreement, instrument or other arrangement pursuant to which Biogen or any Affiliate acquired the right to use such Intellectual Property; (iii) to the Knowledge of Biogen, no Person has advised Biogen or any of its Subsidiaries in writing that it is challenging or threatening to challenge the ownership, use, validity or enforceability of any Intellectual Property owned or used by Biogen or its Subsidiaries; (iv) to the Knowledge of Biogen, no Person is misappropriating, infringing, diluting or otherwise violating any right of Biogen or any of its Subsidiaries with respect to any Intellectual Property owned or used by Biogen or its Subsidiaries; (v) to the Knowledge of Biogen, neither Biogen nor any of its Subsidiaries has received written notice of any pending or threatened claim, order or proceeding with respect to the validity, enforcement or maintenance of any Intellectual Property owned or used by Biogen or its Subsidiaries and, to the Knowledge of Biogen, no Intellectual Property owned or used by Biogen or its Subsidiaries is being used or enforced in a manner that would reasonably be expected to result in the abandonment, cancellation or unenforceability of such Intellectual Property; (vi) to the Knowledge of Biogen, the Intellectual Property owned or used by Biogen or its Subsidiaries has not expired, been cancelled or abandoned and all maintenance and renewal fees necessary to preserve the rights of Biogen in connection with such Intellectual Property have been paid in a timely manner; (vii) neither Biogen nor any of its Subsidiaries has entered into any consent, indemnification, forbearance to sue, settlement agreement, license or other arrangement which reasonably could be expected to provide a third party a defense to patent infringement in connection with any Intellectual Property owned or used by Biogen; (viii) to the Knowledge of Biogen, Biogen and each of its Subsidiaries has implemented commercially reasonable measures to maintain the confidentiality of the Intellectual Property and all other property used in the business of Biogen or its Subsidiaries as presently conducted; (ix) each current and former employee of Biogen or its Subsidiaries who has contributed to or participated in research and development activities has entered into an agreement with Biogen or a Biogen Subsidiary that has accorded Biogen or the Subsidiary full, effective and exclusive ownership rights in and to all tangible or intangible property created thereby; and (x) each employee, officer and director of Biogen or its Subsidiaries has entered into an agreement to maintain the confidential information of Biogen and

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its Subsidiaries and, to the Knowledge of Biogen, all consultants of Biogen or its Subsidiaries who have been in a position to receive any confidential information of Biogen or its Subsidiaries have entered into written agreements with Biogen or its Subsidiaries to maintain the confidentiality of all such information.

        (n)    State Takeover Statutes.    The Board of Directors of Biogen has adopted a resolution or resolutions approving this Agreement, the Merger and the other transactions contemplated hereby, and, assuming the accuracy of IDEC's representation and warranty contained in Section 3.2(q) (without giving effect to the Knowledge qualification contained therein), such approval constitutes approval of the Merger and the other transactions contemplated hereby by the Board of Directors of Biogen under the provisions of Chapter 110F of the Massachusetts General Laws ("Chapter 110F") such that Chapter 110F does not apply to this Agreement, the Merger or the other transactions contemplated hereby. To the Knowledge of Biogen, no state takeover statute other than Chapter 110F (which has been rendered inapplicable) is applicable to the Merger or the other transactions contemplated hereby.

        (o)    Brokers.    Except for fees payable to Goldman, Sachs & Co., no broker, investment banker, financial advisor or other Person, is entitled to any broker's, finder's, financial advisor's or other similar fee or commission in connection with the transactions contemplated by this Agreement based upon arrangements made by or on behalf of Biogen.

        (p)    Opinion of Financial Advisor.    Biogen has received the opinion of its financial advisor, Goldman, Sachs & Co., dated the date of this Agreement, to the effect that, as of such date, the Exchange Ratio is fair, from a financial point of view, to the holders of Biogen Common Stock.

        (q)    Ownership of IDEC Common Stock.    None of Biogen, its Subsidiaries or, to the Knowledge of Biogen without independent investigation, any of their respective Affiliates, (i) beneficially owns (as defined in Rule 13d-3 under the Exchange Act) directly or indirectly, or is party to any agreement, arrangement or understanding for the purpose of acquiring, holding, voting or disposing of, shares of capital stock of IDEC, or (ii) within the preceding thirty-six (36) months, beneficially owned directly or indirectly, or was party to any agreement, arrangement or understanding for the purpose of acquiring, holding, voting or disposing of, shares of capital stock of IDEC.

        (r)    Material Contracts.    

        (i)    For purposes of this Agreement, "Biogen Material Contract" shall mean:

        (1)   any "material contracts" (as such term is used in Item 601(b)(10) of Regulation S-K of the SEC) with respect to Biogen and its Subsidiaries;

        (2)   any Contract to which Biogen or any of its Subsidiaries is a party, which is material to Biogen and its Subsidiaries, taken together, and which (A) contains any covenant limiting or restricting the right of Biogen or any of its Subsidiaries or that would, after the Effective Time, limit or restrict IDEC or any of its Subsidiaries (including the Surviving Corporation and its Subsidiaries) from (x) engaging or competing in any material line of business or in any geographic area or with any Person in any material line of business or (y) making use of any material Intellectual Property owned by or necessary for the conduct of the businesses of Biogen and its Subsidiaries in the manner described in the Biogen SEC Documents filed prior to the date hereof and as they are currently being conducted, (B) relates to the development, co-promotion, commercialization, manufacturing, supply or distribution of any material product, the out-licensing to third parties of Intellectual Property relating to any material product or the in-licensing from third parties of Intellectual Property relating to any material

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product, or (C) adversely affects the right of Biogen or any of its Subsidiaries to use, sell, distribute, manufacture or have manufactured or supply any material product; and

        (3)   any Contract or group of Contracts with a Person (or group of affiliated Persons) to which Biogen or any of its Subsidiaries is a party, the termination or breach of which would be reasonably likely to have a Material Adverse Effect on Biogen.

        (ii)    Schedule.    Section 3.1(r)(ii) of the Biogen Disclosure Schedule sets forth a list of all Biogen Material Contracts as of the date hereof.

        (iii)    No Breach.    All Biogen Material Contracts are valid and in full force and effect and enforceable in accordance with their respective terms, subject to applicable bankruptcy, insolvency, reorganization, moratorium or other laws relating to or affecting the rights and remedies of creditors generally and to general principles of equity (regardless of whether considered in a proceeding in equity or at law), except to the extent that (A) they have previously expired in accordance with their terms or (B) the failure to be in full force and effect, individually or in the aggregate, would not reasonably be likely to have a Material Adverse Effect on Biogen. Neither Biogen nor any of its Subsidiaries, nor, to Biogen's Knowledge, any counterparty to any Biogen Material Contract, has violated any provision of, or committed or failed to perform any act which, with or without notice, lapse of time or both would constitute a default under the provisions of, any Biogen Material Contract, except in each case for those violations and defaults which, individually or in the aggregate, would not reasonably be likely to have a Material Adverse Effect on Biogen.

        (s)    Interested Party Transactions.    Since the date of the Biogen Balance Sheet, no event has occurred that would be required to be reported as a Certain Relationship or Related Transaction pursuant to Statement of Financial Accounting Standards No. 57 or Item 404 of Regulation S-K of the SEC.

        (t)    Biogen Rights Agreement.    Biogen has taken all action so that the execution of this Agreement, the consummation of the Merger and the other transactions contemplated hereby do not and will not result in the grant of any rights to any Person under the Biogen Rights Agreement or enable, require or cause the Biogen Rights to be exercised, distributed or triggered.

        SECTION 3.2    Representations and Warranties of IDEC and Merger Sub.    Except as set forth in the disclosure schedule dated as of the date of this Agreement and executed and delivered by IDEC and Merger Sub to Biogen concurrently with or prior to the execution and delivery by IDEC and Merger Sub of this Agreement (the "IDEC Disclosure Schedule"), IDEC and Merger Sub represent and warrant to Biogen as set forth below. Each exception set forth in the IDEC Disclosure Schedule, and any other information included in the IDEC Disclosure Schedule, is identified by reference to, or has been grouped under a heading referring to, a specific individual subsection of this Agreement and shall be deemed to be disclosed solely for purposes of such subsection, except to the extent that disclosure in one subsection of the IDEC Disclosure Schedule is specifically referred to in another subsection of the IDEC Disclosure Schedule by appropriate cross-reference.

        (a)    Organization, Standing and Corporate Power; Charter Documents; Subsidiaries.    

        (i)    Organization, Standing and Corporate Power.    IDEC and each of its Subsidiaries is a corporation or other legal entity duly organized, validly existing and in good standing (with respect to jurisdictions which recognize such concept) under the laws of the jurisdiction in which it is incorporated or otherwise organized and has the requisite corporate (or similar) power and authority to carry on its business as currently conducted, except for those jurisdictions in which the failure to be so organized, existing or in good standing would not, individually or in the aggregate, reasonably be likely to have a Material Adverse Effect on IDEC. Each of IDEC and its Subsidiaries is duly qualified or licensed to do business and is in

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good standing (with respect to jurisdictions which recognize such concept) in each jurisdiction in which the nature of its business or the ownership, leasing or operation of its properties makes such qualification or licensing necessary, except for those jurisdictions where the failure to be so qualified or licensed or to be in good standing would not, individually or in the aggregate, reasonably be likely to have a Material Adverse Effect on IDEC.

        (ii)    Charter Documents.    IDEC and Merger Sub have delivered or made available to Biogen prior to the execution of this Agreement complete and correct copies of (A) the Amended and Restated Certificate of Incorporation of IDEC (including any certificates of designation), as amended and currently in effect (the "IDEC Charter"), and the by-laws of IDEC, as amended and currently in effect (the "IDEC By-Laws," and, together with the IDEC Charter, the "IDEC Organizational Documents"), and (B) the Articles of Organization and by-laws of Merger Sub in effect on the date of this Agreement and the like organizational documents of each of the IDEC Significant Subsidiaries (as defined in Section 3.2(a)(iii)), as amended and currently in effect (collectively, the "IDEC Subsidiary Organizational Documents"), and each such instrument is in full force and effect. IDEC is not in violation of the IDEC Organizational Documents and no IDEC Significant Subsidiary is in violation of its IDEC Subsidiary Organizational Documents, except, in the case of such IDEC Significant Subsidiary, as would not reasonably be likely to have a Material Adverse Effect on IDEC.

        (iii)    Subsidiaries.    Section 3.2(a)(iii) of the IDEC Disclosure Schedule lists all the Subsidiaries of IDEC which as of the date of this Agreement are significant subsidiaries (as defined in Rule 102 of Regulation S-X of the SEC) (the "IDEC Significant Subsidiaries"). All the outstanding shares of capital stock of, or other equity interests in, each IDEC Significant Subsidiary have been validly issued and are fully paid and nonassessable and are owned directly or indirectly by IDEC, free and clear of all Liens and free of any other restriction (including any restriction on the right to vote, sell or otherwise dispose of such capital stock or other ownership interests).

        (b)    Capital Structure.    

        (i)    The authorized capital stock of IDEC consists of 500,000,000 shares of IDEC Common Stock and 8,000,000 shares of convertible preferred stock, par value $0.001 per share ("IDEC Preferred Stock"). At the close of business on May 31, 2003 (A) 155,257,594 shares of IDEC Common Stock were issued and outstanding; (B) 2,209,493 shares of IDEC Common Stock were held by IDEC in its treasury; (C) 36,214 shares of IDEC Preferred Stock were issued and outstanding, of which 13,221 shares were designated as Series A-2 shares and 22,993 shares were designated as Series A-3 shares, and 2,172,840 shares of IDEC Common Stock were reserved for issuance upon conversion of the IDEC Preferred Stock; (D) 13,934,774 shares of IDEC Common Stock were reserved for issuance upon conversion of IDEC's subordinated convertible notes, due 2019; (E) 8,661,301 shares of IDEC Common Stock were reserved for issuance upon conversion of IDEC's senior convertible notes, due 2032; (F) 1,001,680 shares of IDEC Common Stock were reserved for issuance pursuant to IDEC 1995 Employee Stock Purchase Plan, as amended and restated through February 19, 2003 (the "IDEC Purchase Plan"); (G) 57,256,742 shares of IDEC Common Stock were reserved for issuance pursuant to the IDEC 1988 Stock Option Plan, as amended and restated through January 16, 2001 (the "IDEC 1988 Stock Option Plan"), and the IDEC 1993 Non-Employee Directors Stock Option Plan, as amended and restated through February 19, 2003 (such plans, collectively, the "IDEC Stock Plans"), complete and correct copies of which, in each case as amended, have been filed as exhibits to the IDEC SEC Documents (as defined in Section 3.2(d)(i)) prior to the date of this Agreement or delivered to Biogen; and (H) 1,000,000 shares of IDEC Preferred Stock were designated as Series X Junior Participating Preferred Stock, par value $0.001 per share, and were reserved for issuance upon

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the exercise of preferred stock purchase rights (the "IDEC Rights") issued pursuant to the Amended and Restated IDEC Rights Agreement dated July 26, 2001 between IDEC and Mellon Investor Services LLP, as amended by Amendment No. 1 to Rights Agreement dated