BIOGEN IDEC INC., as Issuer and The Bank of New York Trust Company, N.A., as Trustee First Supplemental Indenture Dated as of March 4, 2008

Exhibit 4.1

BIOGEN IDEC INC.,

as Issuer

and

The Bank of New York Trust Company, N.A.,

as Trustee

First Supplemental Indenture

Dated as of March 4, 2008

TABLE OF CONTENTS


	Page

ARTICLE I. DEFINITIONS		1

SECTION 1.1. Certain Terms Defined in the Indenture		1

SECTION 1.2. Definitions		2

ARTICLE II. FORM AND TERMS OF THE NOTES		5

SECTION 2.1. Form and Dating		5

SECTION 2.2. Terms of the Notes		7

SECTION 2.3. Optional Redemption		10

SECTION 2.4. Repurchase of Notes Upon a Change of Control		10

SECTION 2.5. Limitation on Liens		11

SECTION 2.6. Limitation on Sale and Leaseback Transactions		12

SECTION 2.7. Exempted Liens and Sale and Leaseback Transactions		13

SECTION 2.8. SEC Reports		13


ARTICLE III. MISCELLANEOUS		13

SECTION 3.1. Trust Indenture Act Controls		13

SECTION 3.2. Governing Law		13

SECTION 3.3. Multiple Counterparts		14

SECTION 3.4. Severability		14

SECTION 3.5. Ratification		14

SECTION 3.6. Effectiveness		14

EXHIBIT A— Form of 6.000% Senior Note due 2013		A-1
EXHIBIT B— Form of 6.875% Senior Note due 2018		B-1

FIRST SUPPLEMENTAL INDENTURE

SUPPLEMENTAL INDENTURE (this “First Supplemental Indenture”), dated as of March 4, 2008, between BIOGEN IDEC INC., a Delaware corporation (the “Company”), and The Bank of New York Trust Company, a national association, as Trustee (the “Trustee”).

RECITALS OF THE COMPANY

WHEREAS, the Company and the Trustee executed and delivered an Indenture, dated as of February 26, 2008 (the “Indenture”), to provide for the issuance by the Company from time to time of Securities to be issued in one or mores series as provided in the Indenture;

WHEREAS, the issuance and sale of $450,000,000 aggregate principal amount of a new series of the Securities of the Company designated as its 6.000% Senior Notes due March 1, 2013 (the “Notes due 2013”) and $550,000,000 aggregate principal amount of a new series of the Securities of the Company designated as its 6.875% Senior Notes due March 1, 2018 (the “Notes due 2018”, and together with the Notes due 2013, the “Notes”) have been authorized by resolutions adopted by the Board of Directors of the Company;

WHEREAS, the Company desires to issue and sell $1,000,000,000 aggregate principal amount of the Notes on the date hereof;

WHEREAS, Sections 2.2 and 8.1 of the Indenture provide that the Company, when authorized by a Board Resolution, and the Trustee may amend or supplement the Indenture to provide for the issuance of and to establish the form or terms and conditions of Securities of any Series as permitted by the Indenture;

WHEREAS, the Company desires to establish the form, terms and conditions of the Notes; and

WHEREAS, all things necessary to make this First Supplemental Indenture a valid supplement to the Indenture according to its terms and the terms of the Indenture have been done;

NOW, THEREFORE, for and in consideration of the premises stated herein and the purchase of the Notes by the Holders thereof, the parties hereto hereby enter into this First Supplemental Indenture, for the equal and proportionate benefit of all Holders of the Notes, as follows:

ARTICLE I.

DEFINITIONS

SECTION 1.1. Certain Terms Defined in the Indenture.

For purposes of this Supplemental Indenture, all capitalized terms used but not defined herein shall have the meanings ascribed to such terms in the Indenture, as amended hereby.

SECTION 1.2. Definitions.

For the benefit of the Holders of the Notes, Section 1.1 of the Indenture shall be amended by adding the following new definitions:

“Attributable Debt” means, with respect to a Sale and Leaseback Transaction, an amount equal to the lesser of: (1) the fair market value of the property (as determined in good faith by the Company’s board of directors); and (2) the present value of the total net amount of rent payments to be made under the lease during its remaining term, discounted at the rate of interest set forth or implicit in the terms of the lease, compounded semi-annually. The calculation of the present value of the total net amount of rent payments is subject to adjustments specified in the indenture.

“Bankruptcy Law” means title 11, U.S. Code or any similar foreign, Federal or State law for the relief of debtors.

“Capitalized Lease” means any obligation of a Person to pay rent or other amounts incurred with respect to real property or equipment acquired or leased by such Person and used in its business that is required to be recorded as a capital lease in accordance with generally accepted accounting principles.

“Change of Control” means the occurrence of any of the following: (1) the consummation of any transaction (including, without limitation, any merger or consolidation) the result of which is that any “person” (as such term is used in Section 13(d) of the Exchange Act) (other than the Company or one of its Subsidiaries) becomes the beneficial owner (as defined in Rules 13d-3 and 13d-5 under the Exchange Act), directly or indirectly, of more than 50% of the Voting Stock of the Company or other Voting Stock into which the Voting Stock of the Company is reclassified, consolidated, exchanged or changed, measured by voting power rather than number of shares; (2) the direct or indirect sale, transfer, conveyance or other disposition (other than by way of merger or consolidation), in one or more series of related transactions, of all or substantially all of the assets of the Company and the assets of its Subsidiaries, taken as a whole, to one or more “persons” (as such term is used in Section 13(d) of the Exchange Act) (other than to the Company or one of its Subsidiaries); (3) the Company consolidates with, or merges with or into, any “person” (as such term is used in Section 13(d) of the Exchange Act), or any such person consolidates with, or merges with or into, the Company, in either case, pursuant to a transaction in which any of the Company’s outstanding Voting Stock or the Voting Stock of such other person is converted into or exchanged for cash, securities or other property, other than pursuant to a transaction in which shares of the Company’s Voting Stock outstanding immediately prior to the transaction constitute, or are converted into or exchanged for, a majority of the Voting Stock of the surviving person immediately after giving effect to such transaction; (4) the adoption of a plan relating to the Company’s liquidation or dissolution; or (5) the first day on which a majority of the members of the Board of Directors of the Company are not Continuing Directors.

“Change of Control Triggering Event” means the occurrence of both a Change of Control and a Rating Event.

“Comparable Treasury Issue” means the United States Treasury security or securities selected by an Independent Investment Banker as having an actual or interpolated maturity comparable to the remaining term of the Notes to be redeemed that would be utilized, at the time of selection and in accordance with customary financial practice, in pricing new issues of corporate debt securities of a comparable maturity to the remaining term of such Notes.

“Comparable Treasury Price” means, with respect to any Redemption Date, (A) the average of the Reference Treasury Dealer Quotations for such Redemption Date, after excluding the highest and lowest such Reference Treasury Dealer Quotations, or (B) if the Trustee obtains fewer than four such Reference Treasury Dealer Quotations, the average of all quotations.

“Continuing Director” means, as of any date of determination, any member of the Company’s board of directors who (1) was a member of such board of directors on the date the Notes were issued, (2) was nominated for election to such board of directors with the approval of a committee of the board of directors consisting of a majority of independent Continuing Directors or (3) was nominated for election, elected or appointed to such board of directors with the approval of a majority of the Continuing Directors who were members of such board of directors at the time of such nomination, election or appointment (either by a specific vote or by approval of the Company’s proxy statement in which such member was named as a nominee for election as a director, without objection to such nomination).

“Consolidated Total Assets” means, with respect to any Person as of any date, the amount of total assets as shown on the consolidated balance sheet of such Person for the most recent fiscal quarter for which financial statements have been filed with the Securities and Exchange Commission, prepared in accordance with accounting principles generally accepted in the United States.

“Exchange Act” means the Securities Exchange Act of 1934, as amended.

“Global Notes” means, individually and collectively, each of the Notes in the form of Global Securities issued to the Depositary or its nominee, substantially in the form of Exhibits A and B.

“Indebtedness” of any Person means, without duplication (1) any obligation of such Person for money borrowed, (2) any obligation of such Person evidenced by bonds, debentures, notes or other similar instruments, (3) any reimbursement obligation of such Person in respect of letters of credit or other similar instruments which support financial obligations which would otherwise become Indebtedness, and (4) any obligation of such Person under Capitalized Leases; provided, however, that “Indebtedness” of such Person shall not include any obligation of such Person to any Subsidiary of such Person or to any Person with respect to which such Person is a Subsidiary.

“Independent Investment Banker” means one of the Reference Treasury Dealers appointed by the Trustee after consultation with the Company.

“Investment Grade Rating” means a rating equal to or higher than Baa3 (or the equivalent) by Moody’s and BBB- (or the equivalent) by S&P, and the equivalent investment

grade credit rating from any additional rating agency or Rating Agencies selected by the Company.

“Lien” means any pledge, mortgage, lien, encumbrance or other security interest.

“Moody’s” means ▇▇▇▇▇’▇ Investors Service, Inc. or any successor thereto.

“Person” means any individual, corporation, limited liability company, partnership, joint venture, association, joint stock company, trust, unincorporated organization or government or any agency or political subdivision thereof or other similar entity.

“Property” means any property or asset, whether real, personal or mixed, or tangible or intangible.

“Rating Agencies” means (1) each of Moody’s and S&P, and (2) if any of Moody’s and S&P ceases to rate the Notes or fails to make a rating of the Notes publicly available for reasons outside of the control of the Company, a “nationally recognized statistical rating organization” within the meaning of Rule 15c3-1(c)(2)(vi)(F) under the Exchange Act selected by the Company (as certified by a resolution of the Board of Directors of the Company) and which is reasonably acceptable to the Trustee as a replacement agency for Moody’s or S&P or both of them, as the case may be.

“Rating Event” means (A) with respect to the Notes due 2013, the rating on the Notes due 2013 is lowered by each of the Rating Agencies and the Notes due 2013 are rated below an Investment Grade Rating by each of the Rating Agencies, and (B) with respect to the Notes due 2018, the rating on the Notes due 2018 is lowered by each of the Rating Agencies and the Notes due 2018 are rated below an Investment Grade Rating by each of the Rating Agencies, in either case, on any day during the period commencing on the earlier of the date of the first public notice of the occurrence of a Change of Control or the Company’s intention to effect a Change of Control and ending 60 days following consummation of such Change of Control (which period will be extended so long as the rating of the applicable series of Notes is under publicly announced consideration for a possible downgrade by any of the Rating Agencies).

“Reference Treasury Dealer” means each of ▇▇▇▇▇▇▇ Lynch, Pierce, ▇▇▇▇▇▇ & ▇▇▇▇▇ Incorporated and ▇▇▇▇▇▇▇, Sachs & Co. or their respective affiliates, which are primary U.S. Government securities dealers in The City of New York, and their respective successors plus three other primary U.S. Government securities dealers in The City of New York selected by the Company; provided, however, that if any of the foregoing or their affiliates shall cease to be a primary U.S. Government securities dealer in The City of New York (a “Primary Treasury Dealer”), the Company will substitute therefor another Primary Treasury Dealer.

“Reference Treasury Dealer Quotations” means, with respect to each Reference Treasury Dealer and any Redemption Date, the average, as determined by the Trustee, of the bid and asked prices for the Comparable Treasury Issue (expressed in each case as a percentage of its principal amount) quoted in writing to the Trustee by the Reference Treasury Dealers at 3:30 p.m. New York time on the third Business Day preceding such Redemption Date.

“Sale and Leaseback Transaction” means any arrangement with any Person providing for the leasing by the Company or any Subsidiary of the Company of any Property that has been or is to be sold or transferred by the Company or such Subsidiary, as the case may be, to such Person.

“Subsidiary” of any Person means (1) a corporation, a majority of the outstanding voting stock of which is, at the time, directly or indirectly, owned by such Person by one or more Subsidiaries of such Person, or by such Person and one or more Subsidiaries thereof or (2) any other Person (other than a corporation), including, without limitation, a partnership or joint venture, in which such Person, one or more Subsidiaries thereof or such Person and one or more Subsidiaries thereof, directly or indirectly, at the date of determination thereof, has at least majority ownership interest entitled to vote in the election of directors, managers or trustees thereof (or other Person performing similar functions).

“S&P” means Standard & Poor’s Ratings Services, a division of The ▇▇▇▇▇▇-▇▇▇▇ Companies, Inc., or any successor thereto.

“Treasury Rate” means, with respect to any Redemption Date, the rate per annum equal to the semiannual equivalent yield to maturity or interpolated (on a day count basis) of the Comparable Treasury Issue, assuming a price for the Comparable Treasury Issue (expressed as a percentage of its principal amount) equal to the Comparable Treasury Price for such Redemption Date.

“Voting Stock” means, with respect to any specified “person” (as that term is used in Section 13(d) of the Exchange Act) as of any date, the capital stock of such person that is at the time entitled to vote generally in the election of the board of directors of such person.

ARTICLE II.

FORM AND TERMS OF THE NOTES

SECTION 2.1. Form and Dating.

The Notes due 2013 and the Trustee’s certificate of authentication shall be substantially in the form of Exhibit A attached hereto. The Notes due 2018 and the Trustee’s certificate of authentication shall be substantially in the form of Exhibit B attached hereto. The Notes shall be executed on behalf of the Company by two Officers of the Company or an Officer and an Assistant Secretary of the Company. The Company’s seal shall be impressed, affixed, imprinted or reproduced thereon. The Notes may have notations, legends or endorsements required by law, stock exchange rules or usage. Each Note shall be dated the date of its authentication. The Notes and any beneficial interest in the Notes shall be in minimum denominations of $2,000 and integral multiples of $1,000 in excess thereof.

The terms and notations contained in the Notes shall constitute, and are hereby expressly made, a part of the Indenture as supplemented by this First Supplemental Indenture and the Company and the Trustee, by their execution and delivery of this First Supplemental Indenture, expressly agree to such terms and provisions and to be bound thereby.

(a) Global Notes. The Notes of each series designated herein shall be issued initially in the form of one or more fully registered Global Securities, which shall be deposited on behalf of the purchasers of the Notes represented thereby with The Depository Trust Company, New York, New York (the “Depositary”) and registered in the name of Cede & Co., the Depositary’s nominee, duly executed by the Company, authenticated by the Trustee and with guarantees endorsed thereon as hereinafter provided. The aggregate principal amount of outstanding Notes may from time to time be increased or decreased by adjustments made on the records of the Trustee and the Depositary or its nominee as hereinafter provided.

The Global Notes may not be transferred except by the Depositary, in whole and not in part, to another nominee of the Depositary or to a successor of the Depositary or its nominee. If at any time the Depositary for the Notes notifies the Company that the Depositary is unwilling to continue as Depositary for the Global Notes or ceases to be a clearing agency, or if the Company so elects or if there is an Event of Default under the Notes, then the Company shall execute, and the Trustee shall, upon receipt of a Company Order for authentication, authenticate and deliver, Definitive Notes in an aggregate principal amount equal to the principal amount of the Global Notes in exchange for such Global Note, which the Depositary will distribute to its participants.

(b) Book-Entry Provisions. This Section 2.1(b) shall apply only to the Global Notes deposited with or on behalf of the Depositary.

The Company shall execute and the Trustee shall, in accordance with this Section 2.1(b), authenticate and deliver the Global Notes that shall be registered in the name of the Depositary or the nominee of the Depositary and shall be delivered by the Trustee to the Depositary or pursuant to the Depositary’s instructions.

Depositary Participants shall have no rights either under the Indenture or with respect to any Global Notes held on their behalf by the Depositary or under such Global Notes. The Depositary shall be treated by the Company, the Trustee and any agent of the Company or the Trustee as the absolute owner of such Global Note for all purposes under the Indenture. Notwithstanding the foregoing, nothing herein shall prevent the Company or the Trustee from giving effect to any written certification, proxy or other authorization furnished by the Depositary or impair, as between the Depositary and the Depository Participants, the operation of customary practices of such Depositary governing the exercise of the rights of an owner of a beneficial interest in the Global Notes.

(c) Definitive Notes. Notes issued in certificated form shall be substantially in the form of Exhibit A or Exhibit B, as applicable, attached hereto, but without including the text referred to therein as applying only to Global Notes. Except as provided above in subsection (a), owners of beneficial interests in the Global Notes will not be entitled to receive physical delivery of certificated Notes.

(d) Transfer and Exchange of the Notes. The transfer and exchange of beneficial interests in the Global Notes shall be effected through the Depositary, in accordance with the Indenture and the procedures of the Depositary therefor. Beneficial

interests in the Global Notes may be transferred to Persons who take delivery thereof in the form of a beneficial interest in the Global Notes.

(e) Paying Agent. The Company appoints the Trustee as the initial agent of the Company for the payment of the principal of (and premium, if any) and interest on the Notes and the Corporate Trust Office of the Trustee in the Borough of Manhattan, the City of New York, be and hereby is, designated as the office or agency where the Notes may be presented for payment and where notices to or demands upon the Company in respect of the Notes and the Indenture pursuant to which the Notes are to be issued may be served.

SECTION 2.2. Terms of the Notes.

The following terms relating to the Notes are hereby established:

(a) Title. The Notes due 2013 shall constitute a series of Securities having the title “6.000% Senior Notes due 2013” and the Notes due 2018 shall constitute a separate series of Securities having the title “6.875% Senior Notes due 2018.”

(b) Principal Amount. The aggregate principal amount of the Notes due 2013 that may be initially authenticated and delivered under the Indenture (except for Notes due 2013 authenticated and delivered upon registration of, transfer of, or in exchange for, or in lieu of, other Notes due 2013 pursuant to Sections 2.7, 2.8, 2.11, 3.6 or 8.5 of the Indenture) shall be $450,000,000. The aggregate principal amount of the Notes due 2018 that may be initially authenticated and delivered under the Indenture (except for Notes due 2018 authenticated and delivered upon registration of, transfer of, or in exchange for, or in lieu of, other Notes due 2018 pursuant to Sections 2.7, 2.8, 2.11, 3.6 or 8.5 of the Indenture) shall be $550,000,000. The Company may from time to time, without the consent of the Holders of Notes of either series, issue additional Notes (in any such case “Additional Notes”) of either series having the same ranking and the same interest rate, maturity and other terms as the Notes of that series. Any additional Notes of a series and the existing Notes of that series will constitute a single series under the Indenture and all references to the relevant Notes shall include the Additional Notes unless the context otherwise requires.

(c) Maturity Date. The entire outstanding principal of the Notes due 2013 shall be payable on March 1, 2013, and the entire outstanding principal of the Notes due 2018 shall be payable on March 1, 2018.

(d) Interest Rate. (i) The rate at which the Notes due 2013 shall bear interest shall be 6.000% per annum and the rate at which the Notes due 2018 shall bear interest shall be 6.875% per annum, in each case, subject to Section 2.2(d)(ii); the date from which interest shall accrue on the Notes shall be March 4, 2008, or the most recent Interest Payment Date to which interest has been paid or provided for; the Interest Payment Dates for the Notes shall be March 1 and September 1 of each year, beginning September 1, 2008; the interest so payable, and punctually paid or duly provided for, on any Interest Payment Date, will be paid, in immediately available funds, to the Persons in

whose names the Notes (or one or more predecessor Securities) is registered at the close of business on the Regular Record Date for such interest, which shall be February 15 or August 15, as the case may be, next preceding such Interest Payment Date. Any such interest not punctually paid or duly provided for shall forthwith cease to be payable to the respective Holders on such Regular Record Date, and such defaulted interest, may be paid to the Persons in whose names the Notes (or one or more predecessor Securities) is registered at the close of business on a special record date for the payment of such defaulted interest to be fixed by the Trustee, notice whereof shall be given to Holders of Notes not less than 15 days prior to such special record date, or may be paid at any time in any other lawful manner not inconsistent with requirements of any securities exchange on which the Notes may be listed, and upon such notice as may be required by such exchange, all as more fully provided in said Indenture. Payment of principal and interest on this Note will be made at the Corporate Trust Office of the Trustee or such other office or agency of the Company as may be designated for such purpose, in such coin or currency of the United States of America as at the time of payment is legal tender for payment of public and private debts; provided, however, that each installment of interest and principal on this Notes may at the Company’s option be paid in immediately available funds by transfer to an account maintained by the payee located in the United States.

(ii) The interest rate payable on the Notes of each series shall be subject to adjustment from time to time if either Moody’s or S&P downgrades (or subsequently upgrades) the debt rating assigned to the Notes in the manner described below.

If the rating from Moody’s of the Notes of a series is decreased to a rating set forth in the immediately following table, the interest rate payable on the Notes of such series will increase from the interest rate payable on the Notes of such series on the date of their issuance by the percentage set forth opposite that rating:


Rating	Percentage
Ba1		0.25	%
Ba2		0.50	%
Ba3		0.75	%
B1 or below		1.00	%

If the rating from S&P of the Notes of a series is decreased to a rating set forth in the immediately following table, the interest rate payable on the Notes of such series will increase from the interest rate payable on the Notes of such series on the date of their issuance by the percentage set forth opposite that rating:


Rating	Percentage
BB+		0.25	%
BB		0.50	%
BB-		0.75	%
B+ or below		1.00	%

If at any time the interest rate payable on the Notes of a series has been adjusted upward and either Moody’s or S&P, as the case may be, subsequently increases its rating of the Notes of that series to any of the threshold ratings set forth above, the interest rate payable on the Notes of such series will be decreased such that the interest rate payable on the Notes of such series equals the interest rate payable on the Notes of such series on the date of their issuance plus the percentages set forth opposite the ratings from the tables above in effect immediately following the increase. If Moody’s subsequently increases its rating of the Notes of a series to Baa3 or higher and S&P increases its rating to BBB- or higher the interest rate payable on the Notes of such series will be decreased to the interest rate payable on the Notes of such series on the date of their issuance. In addition, the interest rates on the Notes of each series shall permanently cease to be subject to any adjustment described above (notwithstanding any subsequent decrease in the ratings by either or both rating agencies) if the Notes of such series become rated Baa1 and A- or higher by Moody’s and S&P, respectively (or one of these ratings if only rated by one rating agency).

Each adjustment required by any decrease or increase in a rating set forth above, whether occasioned by the action of Moody’s or S&P, shall be made independent of any and all other adjustments. In no event shall (1) the interest rate payable on the Notes of a series be reduced to below the interest rate payable on the Notes of such series on the date of their issuance or (2) the total increase in the interest rate payable on the Notes of a series exceed 2.00% above the interest rate payable on the Notes of such series on the date of their issuance.

If either Moody’s or S&P ceases to provide a rating of the Notes of a series, any subsequent increase or decrease in the interest rate payable on the Notes of such series necessitated by a reduction or increase in the rating by the agency continuing to provide the rating shall be twice the percentage set forth in the applicable table above. No adjustments in the interest rate payable on the Notes of either series shall be made solely as a result of either Moody’s or S&P ceasing to provide a rating. If both Moody’s and S&P cease to provide a rating of the Notes of a series, the interest rate payable on the Notes of such series will increase to, or remain at, as the case may be, 2.00% above the interest rate payable on the Notes of such series on the date of their issuance.

Any interest rate increase or decrease described above will take effect from the first day of the interest period during which a rating change requires an adjustment in the interest rate.

If the interest rate payable on the Notes of a series is increased as described in this Section 2.2(d)(ii), then the term “interest,” as used in this First Supplemental Indenture, the Indenture and the Notes of the applicable series, will be deemed to include any such additional interest unless the context otherwise requires.

(e) Currency. The currency of denomination of the Notes is United States Dollars. Payment of principal of and interest and premium, if any, on the Notes will be made in United States Dollars.

SECTION 2.3. Optional Redemption.

(a) The provisions of Article 3 of the Indenture shall apply to the Notes.

(b) At any time and from time to time, the Notes of each series will be redeemable, as a whole or in part, at the Company’s option, on at least 30 days, but not more than 60 days, prior notice mailed to the registered address of each holder of the Notes of the applicable series, at a redemption price equal to the greater of (i) 100% of principal amount of the Notes to be redeemed, or (ii) the sum of the present values of the remaining scheduled payments of interest and principal thereon (exclusive of interest accrued and unpaid to, but not including, the date of redemption) discounted to the date of redemption on a semiannual basis, assuming a 360-day year consisting of twelve 30-day months, at the Treasury Rate plus 50 basis points, plus, in either case, accrued and unpaid interest to, but not including, the date of redemption.

(c) On and after the Redemption Date for the Notes of either series, interest will cease to accrue on the Notes of such series or any portion thereof called for redemption, unless the Company defaults in the payment of the redemption price. On or before the Redemption Date for the Notes of such series, the Company will deposit with a Paying Agent, or the Trustee, funds sufficient to pay the redemption price of the Notes to be redeemed on such date. If less than all of the Notes of a series are to be redeemed, the Notes of that series to be redeemed will be selected by the Trustee by such method as the Trustee deems fair and appropriate.

SECTION 2.4. Repurchase of Notes Upon a Change of Control.

(a) If a Change of Control Triggering Event occurs with respect to the Notes of a series, unless the Company shall have exercised its option to redeem the Notes of such series as described in Section 2.3 of this First Supplemental Indenture, the Company shall be required to make an offer (the “Change of Control Offer”) to each holder of the Notes of such series to repurchase all or any part (equal to $2,000 or an integral multiple of $1,000 in excess thereof) of such Holder’s Notes of such series on the terms set forth in this Section 2.4 and in the Notes. In the Change of Control Offer, the Company shall offer payment in cash equal to 101% of the aggregate principal amount of Notes repurchased, plus accrued and unpaid interest, if any, on the Notes repurchased to but not including the date of repurchase (the “Change of Control Payment”). With respect to the Notes of each series, within 30 days following any Change of Control Triggering Event or, at the option of the Company, prior to any Change of Control, but after the public announcement of the transaction that constitutes or may constitute the Change of Control, the Company shall mail a notice to Holders of Notes of the applicable series describing the transaction that constitutes or may constitute the Change of Control Triggering Event and offering to repurchase the Notes of such series on the date specified in the notice, which date shall be no earlier than 30 days and no later than 60 days from the date such notice is mailed or, if the notice is mailed prior to the Change of Control, no earlier than 30 days and no later than 60 days from the date on which the Change of Control Triggering Event occurs (the “Change of Control Payment Date”). The notice shall, if mailed prior to the date of consummation of the Change of Control, state that the offer to

purchase is conditioned on the Change of Control Triggering Event occurring on or prior to the Change of Control Payment Date.

(b) On the Change of Control Payment Date, the Company shall, to the extent lawful:

(1) accept for payment all Notes or portions of Notes properly tendered pursuant to the Change of Control Offer;

(2) deposit with the Paying Agent an amount equal to the Change of Control Payment in respect of all Notes or portions of Notes properly tendered; and

(3) deliver or cause to be delivered to the Trustee the Notes properly accepted together with an Officers’ Certificate stating the aggregate principal amount of Notes or portions of Notes being repurchased.

(c) The Company shall not be required to make a Change of Control Offer upon the occurrence of a Change of Control Triggering Event if a third party makes such an offer in the manner, at the times and otherwise in compliance with the requirements for an offer made by the Company and the third party repurchases all Notes properly tendered and not withdrawn under its offer. In addition, the Company shall not repurchase any Notes if there has occurred and is continuing on the Change of Control Payment Date an Event of Default under the Indenture, other than an Event of Default arising as a result of a default in the payment of the Change of Control Payment upon a Change of Control Triggering Event.

(d) The Company shall comply in all material respects with the requirements of Rule 14e-1 under the Exchange Act and any other securities laws and regulations thereunder to the extent such laws and regulations are applicable in connection with the repurchase of the Notes as a result of a Change of Control Triggering Event. To the extent that the provisions of any such securities laws or regulations conflict with the Change of Control Offer provisions of the Notes, the Company shall comply with the applicable securities laws and regulations and shall not be deemed to have breached its obligations under the Change of Control Offer provisions of the Notes by virtue of any such conflict.

SECTION 2.5. Limitation on Liens.

For the benefit of the Holders of the Notes, a new Section 4.7 shall be added to the Indenture as follows:

“Other than as provided in Section 4.9, the Company shall not, and shall not permit any Subsidiaries of the Company to, create or assume any Indebtedness secured by any Lien on any of the Company’s or such Subsidiary’s respective Properties unless the Notes are secured by such Lien equally and ratably with, or prior to, the Indebtedness secured by such Lien. This restriction does not apply to Indebtedness that is secured by (i) Liens existing on the date of the issuance of the Notes; (ii) Liens securing only the Notes; (iii) Liens on Property or shares of

stock in respect of Indebtedness of a Person existing at the time such Person becomes a Subsidiary of the Company or is merged into or consolidated with, or its assets are acquired by, the Company or any Subsidiary of the Company (provided that such Lien was not incurred in anticipation of such transaction and was in existence prior to such transaction) so long as such Lien does not extend to any other Property and the Indebtedness so secured is not increased; (iv) Liens to secure Indebtedness incurred for the purpose of all or any part of a Property’s purchase price or cost of construction or additions, repairs, alterations, or other improvements; provided that (1) the principal amount of any Indebtedness secured by such Lien does not exceed 100% of such Property’s purchase price or cost, (2) such Lien does not extend to or cover any other Property other than the Property so purchased, constructed or on which such additions, repairs, alterations or other improvements were so made, and (3) such Lien is incurred prior to or within 270 days after the acquisition of such Property or the completion of construction or such additions, repairs, alterations or other improvements and the full operation of such Property thereafter; (v) Liens in favor of the United States or any state thereof, or any instrumentality of either, to secure certain payments pursuant to any contract or statute; (vi) Liens for taxes or assessments or other governmental charges or levies which are not overdue for a period exceeding 60 days unless such Liens are being contested in good faith and for which adequate reserves are being maintained, to the extent required by generally accepted accounting principles; (vii) title exceptions, easements, licenses, leases and other similar Liens that are not consensual and that do not materially impair the use of the Property subject thereto; (viii) Liens to secure obligations under worker’s compensation laws, unemployment compensation, old-age pensions and other social security benefits or similar legislation; (ix) Liens arising out of legal proceedings, including Liens arising out of judgments or awards; (x) warehousemen’s, materialmen’s, carrier’s, landlord’s and other similar Liens for sums not overdue for a period exceeding 60 days unless such Liens are being contested in good faith and for which adequate reserves are being maintained, to the extent required by generally accepted accounting principles; (xi) Liens incurred to secure the performance of statutory obligations, surety or appeal bonds, performance or return-of-money bonds, insurance, self-insurance or other obligations of a like nature incurred in the ordinary course of business; (xii) Liens that are rights of set-off relating to the establishment of depository relations with banks not given in connection with the issuance of Indebtedness; (xiii) Liens on the assets of a special purpose Subsidiary resulting from securitization transactions with respect to accounts receivable, royalties and similar assets included in such securitization transactions; or (xiv) Liens to secure any extension, renewal, refinancing or refunding (or successive extensions, renewals, refinancings or refundings), in whole or in part, of any Indebtedness secured by Liens referred to in clauses (i) to (xiii) or Liens created in connection with any amendment, consent or waiver relating to such Indebtedness, so long as such Lien does not extend to any other Property and the Indebtedness so secured does not exceed the fair market value (as determined by the Company’s board of directors) of the assets subject to such Liens at the time of such extension, renewal, refinancing or refunding, or such amendment, consent or waiver, as the case may be.”

SECTION 2.6. Limitation on Sale and Leaseback Transactions.

For the benefit of the Holders of the Notes a new Section 4.8 shall be added to the Indenture as follows:

“Other than as provided under Section 4.9, the Company shall not, and shall not permit any of its Subsidiaries to, enter into any Sale and Leaseback Transaction with respect to any of the Company’s or such Subsidiary’s respective Properties, the acquisition or completion of construction and commencement of full operations of which has occurred more than 270 days prior thereto, unless (i) the lease is for a period not in excess of five years, including renewal rights; or (ii) the Company or the Subsidiary, prior to or within 270 days after the sale of such Property in connection with the Sale and Leaseback Transaction is completed, applies the net cash proceeds of the sale of the Property leased to (1) the retirement of the Notes or debt of the Company ranking equally with the Notes or to the retirement of any debt of a Subsidiary of the Company, or (2) the acquisition of different property, facilities or equipment or the expansion of the Company’s existing business, including the acquisition of other businesses.”

SECTION 2.7. Exempted Liens and Sale and Leaseback Transactions.

For the benefit of the Holders of the Notes a new Section 4.9 shall be added to the Indenture as follows:

“Notwithstanding the restrictions described under Sections 4.7 or 4.8, the Company or any Subsidiary of the Company may create or assume any Liens or enter into any Sale and Leaseback Transactions not otherwise permitted as described above, if the sum of the following does not exceed 10% of Consolidated Total Assets (i) the outstanding Indebtedness secured by such Liens (not including any Liens permitted under Section 4.7 which amount does not include any Liens permitted under the provisions of this Section 4.9); plus (ii) all Attributable Debt in respect of such Sale and Leaseback Transaction entered into (not including any Sale and Leaseback Transactions permitted under Section 4.8 which amount does not include any Sale and Leaseback Transactions permitted under the provisions of this Section 4.9), measured, in each case, at the time such Lien is incurred or any such Sale and Leaseback Transaction is entered into by the Company or such Subsidiary of the Company.”

ARTICLE III.

MISCELLANEOUS

SECTION 3.1. Trust Indenture Act Controls.

If any provision of this First Supplemental Indenture limits, qualifies or conflicts with another provision which is required to be included in this First Supplemental Indenture by the TIA, the required provision shall control. If any provision of this First Supplemental Indenture modifies or excludes any provision of the TIA which may be so modified or excluded, the latter provision shall be deemed to apply to this First Supplemental Indenture as so modified or to be excluded, as the case may be.

SECTION 3.2. Governing Law.

This First Supplemental Indenture and the Notes shall be governed by and construed in accordance with the laws of the State of New York, as applied to contracts made and performed within the State of New York without regard to principles of conflicts of law.

SECTION 3.3. Multiple Counterparts.

The parties may sign multiple counterparts of this First Supplemental Indenture. Each signed counterpart shall be deemed an original, but all of them together represent one and the same First Supplemental Indenture.

SECTION 3.4. Severability.

Each provision of this First Supplemental Indenture shall be considered separable and if for any reason any provision which is not essential to the effectuation of the basic purpose of this First Supplemental Indenture or the Notes shall be invalid, illegal or unenforceable, the validity, legality and enforceability of the remaining provisions shall not in any way be affected or impaired thereby and a Holder shall have no claim therefor against any party hereto.

SECTION 3.5. Ratification.

The Indenture, as supplemented and amended by this First Supplemental Indenture, is in all respects ratified and confirmed. The Indenture and this First Supplemental Indenture shall be read, taken and construed as one and the same instrument. All provisions included in this First Supplemental Indenture supersede any conflicting provisions included in the Indenture unless not permitted by law. The Trustee accepts the trusts created by the Indenture, as supplemented by this First Supplemental Indenture, and agrees to perform the same upon the terms and conditions of the Indenture, as supplemented by this First Supplemental Indenture.

SECTION 3.6. Effectiveness.

The provisions of this First Supplemental Indenture shall become effective as of the date hereof.

[Remainder of page intentionally left blank.]

IN WITNESS WHEREOF, the parties hereto have caused this First Supplemental Indenture to be duly executed as of the date first above written.


		BIOGEN IDEC INC.

		By:		/s/ ▇▇▇▇▇▇▇ ▇. ▇▇▇▇▇▇
				Name: ▇▇▇▇▇▇▇ ▇. ▇▇▇▇▇▇
				Title: Vice President and Treasurer

		THE BANK OF NEW YORK TRUST COMPANY, N.A., as Trustee

		By:		/s/ ▇▇▇▇▇▇▇▇ ▇▇▇▇▇▇▇
				Name: ▇▇▇▇▇▇▇▇ ▇▇▇▇▇▇▇
				Title: Assistant Vice President

EXHIBIT A

Form of 6.000% Senior Note due 2013

THIS NOTE IS A GLOBAL SECURITY WITHIN THE MEANING OF THE INDENTURE HEREINAFTER REFERRED TO AND IS REGISTERED IN THE NAME OF THE DEPOSITARY OR A NOMINEE OF THE DEPOSITARY. THIS NOTE IS EXCHANGEABLE FOR NOTES REGISTERED IN THE NAME OF A PERSON OTHER THAN THE DEPOSITARY OR ITS NOMINEE ONLY IN THE LIMITED CIRCUMSTANCES DESCRIBED IN THE INDENTURE, AND MAY NOT BE TRANSFERRED EXCEPT AS A WHOLE BY THE DEPOSITARY TO A NOMINEE OF THE DEPOSITARY, BY A NOMINEE OF THE DEPOSITARY TO THE DEPOSITARY OR ANOTHER NOMINEE OF THE DEPOSITARY OR BY THE DEPOSITARY OR ANY SUCH NOMINEE TO A SUCCESSOR DEPOSITARY OR A NOMINEE OF SUCH A SUCCESSOR DEPOSITARY.

BIOGEN IDEC INC.

6.000% Senior Note due 2013



REGISTERED		PRINCIPAL AMOUNT
No. Specimen		$450,000,000

CUSIP: ▇▇▇▇▇▇▇▇▇
ISIN: US09062XAA19

Biogen Idec Inc., a Delaware corporation (herein called the “Company,” which term includes any successor Person under the Indenture hereinafter referred to), for value received, hereby promises to pay to Cede & Co., or registered assigns, the principal sum of $450,000,000 on March 1, 2013 (the “Maturity Date”) (except to the extent redeemed or repaid prior to the Maturity Date) and to pay interest thereon from March 4, 2008 (the “Original Issue Date”) or from the most recent Interest Payment Date to which interest has been paid or duly provided for semi-annually at the rate of 6.000% per annum, on March 1 and September 1 (each such date, an “Interest Payment Date”), commencing September 1, 2008, until the principal hereof is paid or made available for payment.

Subject to the limitations set forth in Section 2.2(d)(ii) of the First Supplemental Indenture (as defined herein), the interest rate payable on the Notes (as defined herein) will be subject to adjustment from time to time, on the terms set forth in the Indenture, if either Moody’s or S&P downgrades (or subsequently upgrades) the debt rating assigned to the Notes. If the interest rate payable on this Note is increased in accordance with the terms hereof and the Indenture, then the term “interest,” as used in this Note and the Indenture, will be deemed to include any such additional interest unless the context otherwise requires.

Payment of Interest. The interest so payable, and punctually paid or made available for payment, on any Interest Payment Date, will, as provided in the Indenture, be paid, in immediately available funds, to the Person in whose name this Note (or one or more

A-1

Predecessor Securities) is registered at the close of business on February 15 or August 15 (whether or not a Business Day, as defined in the Indenture), as the case may be, next preceding such Interest Payment Date (the “Regular Record Date”). Any such interest not punctually paid or duly provided for (“Defaulted Interest”) will forthwith cease to be payable to the Holder on such Regular Record Date, and such Defaulted Interest, may be paid to the Person in whose name this Note (or one or more Predecessor Securities) is registered at the close of business on a special record date (the “Special Record Date”) for the payment of such Defaulted Interest to be fixed by the Trustee, notice whereof shall be given to Holders of Notes not less than fifteen days prior to such Special Record Date, or may be paid at any time in any other lawful manner not inconsistent with requirements of any securities exchange on which the Notes may be listed, and upon such notice as may be required by such exchange, all as more fully provided in said Indenture.

Place of Payment. Payment of principal, premium, if any, and interest on this Note will be made at the Corporate Trust Office of the Trustee or such other office or agency of the Company as may be designated for such purpose, in such coin or currency of the United States of America as at the time of payment is legal tender for payment of public and private debts; provided, however, that each installment of interest, premium, if any, and principal on this Note may at the Company’s option be paid in immediately available funds by transfer to an account maintained by the payee located in the United States.

Time of Payment. In any case where any Interest Payment Date, the Maturity Date or any date fixed for redemption or repayment of the Notes shall not be a Business Day, then (notwithstanding any other provision of the Indenture or this Note), payment of principal or interest, if any, need not be made on such date, but may be made on the next succeeding Business Day with the same force and effect as if made on such Interest Payment Date, the Maturity Date or the date so fixed for redemption or repayment, and no interest shall accrue in respect of the delay.

General. This Note is one of a duly authorized issue of Securities of the Company, issued and to be issued in one or more series under an indenture (the “Base Indenture”), dated as of February 26, 2008, between the Company and The Bank of New York Trust Company, N.A. (herein called the “Trustee,” which term includes any successor trustee under the Indenture with respect to a series of which this Note is a part), as supplemented by a First Supplemental Indenture thereto, dated as of March 4, 2008 (the “First Supplemental Indenture” and, together with the Base Indenture, the “Indenture”). Reference is hereby made to the Indenture for a statement of the respective rights, limitations of rights, duties and immunities thereunder of the Company, the Trustee and the Holders of the Securities, and of the terms upon which the Securities are, and are to be, authenticated and delivered. This Note is one of a duly authorized series of Securities designated as “6.000% Senior Notes due 2013” (collectively, the “Notes”), initially limited in aggregate principal amount to $450,000,000.

Further Issuance. The Company may from time to time, without the consent of the Holders of the Notes, issue additional Securities (the “Additional Securities”) of this series having the same ranking and the same interest rate, maturity and other terms as the Notes. Any Additional Securities of this series and the Notes will constitute a single series under the

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Indenture and all references to the Notes shall include the Additional Securities unless the context otherwise requires.

Events of Default. If an Event of Default with respect to the Notes shall have occurred and be continuing, the principal of the Notes may be declared due and payable in the manner and with the effect provided in the Indenture.

Sinking Fund. The Notes are not subject to any sinking fund.

Optional Redemption. The Notes will be redeemable at any time, at the option of the Company, in whole or from time to time in part, upon not less than 30 nor more than 60 days’ prior notice, on any date prior to their Maturity at a redemption price, calculated pursuant to the Indenture, which includes accrued interest thereon, if any, to, but not including, the Redemption Date. In the case of any partial redemption, selection of the Notes for redemption will be made by the Trustee by such methods, as the Trustee in its sole discretion shall deem fair and appropriate. If any Note is to be redeemed in part only, the notice of redemption relating to such Note shall state the portion of the principal amount thereof to be redeemed. A new Note in principal amount equal to the unredeemed portion thereof will be issued in the name of the Holder thereof upon cancellation of this Note.

Repurchase upon a Change of Control Triggering Event. Upon the occurrence of a Change of Control Triggering Event with respect to the Notes, the Company shall be required to make an offer to repurchase the Notes on the terms set forth in the Indenture.

Defeasance and Covenant Defeasance. The Indenture contains provisions for defeasance at any time of (a) the entire indebtedness of the Company on this Note and (b) certain restrictive covenants and the related Defaults and Events of Default, upon compliance by the Company with certain conditions set forth therein, which provisions apply to this Note.

Modification and Waivers; Obligations of the Company Absolute. The Indenture permits, with certain exceptions as therein provided, the amendment thereof and the modification of the rights and obligations of the Company and the rights of the Holders of the Securities of each series. Such amendment may be effected under the Indenture at any time by the Company, and the Trustee with the consent of the Holders of not less than a majority in aggregate principal amount of the outstanding Notes of each series affected thereby. The Indenture also contains provisions permitting the Holders of not less than a majority in aggregate principal amount of the Securities at the time outstanding, on behalf of the Holders of all outstanding Securities, to waive compliance by the Company with certain provisions of the Indenture. Furthermore, provisions in the Indenture permit the Holders of not less than a majority in aggregate principal amount of the outstanding Securities of individual series to waive on behalf of all of the Holders of Securities of such individual series certain past defaults under the Indenture and their consequences. Any such consent or waiver shall be conclusive and binding upon the Holder of this Note and upon all future Holders of this Note and of any Note issued upon the registration of transfer hereof or in exchange hereof or in lieu hereof, whether or not notation of such consent or waiver is made upon this Note.

A-3

No reference herein to the Indenture and no provision of this Note or of the Indenture shall alter or impair the obligation of the Company, which is absolute and unconditional, to pay the principal of and interest on this Note at the time, place, and rate, and in the coin or currency, herein prescribed.

Limitation on Suits. As set forth in, and subject to, the provisions of the Indenture, no Holder of any Note will have any right to institute any proceeding with respect to the Indenture or for any remedy thereunder, unless such Holder shall have previously given to the Trustee written notice of a continuing Event of Default with respect to this series, the Holders of not less than 25% in principal amount of the outstanding Notes shall have made written request, and offered reasonable indemnity, to the Trustee to institute such proceedings as trustee, and the Trustee shall not have received from the Holders of a majority in principal amount of the outstanding Notes a direction inconsistent with such request and shall have failed to institute such proceeding within 60 days; provided, however, that such limitations do not apply to a suit instituted by the Holder hereof for the enforcement of payment of the principal of or interest on this Note on or after the respective due dates expressed herein.

Authorized Denominations. The Notes are issuable only in registered form without coupons in denominations of $2,000 or any integral multiple of $1,000 in excess thereof.

Registration of Transfer or Exchange. As provided in the Indenture and subject to certain limitations herein and therein set forth, the transfer of this Note is registrable in the register of the Notes maintained by the Registrar upon surrender of this Note for registration of transfer, at the office or agency of the Company in any place where the principal of and interest on this Note are payable, duly endorsed by, or accompanied by a written instrument of transfer in form satisfactory to the Company and the Registrar, duly executed by the Holder hereof or his attorney duly authorized in writing, and thereupon one or more new Notes, of authorized denominations and for the same aggregate principal amount, will be issued to the designated transferee or transferees.

As provided in the Indenture and subject to certain limitations herein and therein set forth, the Notes are exchangeable for a like aggregate principal amount of Notes of different authorized denominations, as requested by the Holders surrendering the same.

No service charge shall be made for any such registration of transfer or exchange, but the Company may require payment of a sum sufficient to cover any tax or other governmental charge payable in connection therewith.

Prior to due presentment of this Note for registration of transfer, the Company, the Trustee and any agent of the Company or the Trustee may treat the Holder as the owner hereof for all purposes, whether or not this Note be overdue, and neither the Company, the Trustee nor any such agent shall be affected by notice to the contrary.

Defined Terms. All terms used in this Note, which are defined in the Indenture and are not otherwise defined herein, shall have the meanings assigned to them in the Indenture.

A-4

Governing Law. This Note shall be governed by and construed in accordance with the laws of the State of New York, as applied to contracts made and performed within the State of New York without regard to principles of conflicts of law.

Unless the certificate of authentication hereon has been executed by the Trustee by manual signature, this Note shall not be entitled to any benefit under the Indenture or be valid or obligatory for any purpose.

A-5

IN WITNESS WHEREOF, the Company has caused this instrument to be duly executed and its seal to be hereunto affixed and attested.

Dated: March 4, 2008


	BIOGEN IDEC INC.
	By:
		Name:
		Title:

Attest:


By:
		Name:
		Title:

A-6

TRUSTEE’S CERTIFICATE OF AUTHENTICATION

This is one of the Notes of the series designated and referred to in the within-mentioned Indenture, as such is supplemented by the within-mentioned First Supplemental Indenture.


	THE BANK OF NEW YORK TRUST COMPANY, N.A., as Trustee
	By:
		Name:
		Title:

Dated: March 4, 2008

A-7

ASSIGNMENT

FOR VALUE RECEIVED, the undersigned hereby sell(s), assign(s) and transfer(s) unto

PLEASE INSERT SOCIAL SECURITY NUMBER OR OTHER IDENTIFYING NUMBER OF ASSIGNEE

(Please print or typewrite name and address,
including postal zip code, of assignee)

the within Note and all rights thereunder, and hereby irrevocably constitutes and appoints

to transfer said Note on the books of the Trustee, with full power of substitution in the premises.


Dated:
						NOTICE: The signature to this assignment must correspond with the name as written upon the face of the within Note in every particular, without alteration or enlargement or any change whatsoever.


Signature Guarantee

A-8

EXHIBIT B

Form of 6.875% Senior Note due 2018

BIOGEN IDEC INC.

6.875% Senior Note due 2018


REGISTERED		$550,000,000
No. Specimen

CUSIP: ▇▇▇▇▇▇▇▇▇
ISIN: US09062XAB91

Biogen Idec Inc., a Delaware corporation (herein called the “Company,” which term includes any successor Person under the Indenture hereinafter referred to), for value received, hereby promises to pay to Cede & Co., or registered assigns, the principal sum of $550,000,000 on March 1, 2018 (the “Maturity Date”) (except to the extent redeemed or repaid prior to the Maturity Date) and to pay interest thereon from March 4, 2008 (the “Original Issue Date”) or from the most recent Interest Payment Date to which interest has been paid or duly provided for semi-annually at the rate of 6.875% per annum, on March 1 and September 1 (each such date, an “Interest Payment Date”), commencing September 1, 2008, until the principal hereof is paid or made available for payment.

If the interest rate payable on this Note is increased in accordance with the terms hereof and the Indenture, then the term “interest,” as used in this Note and the Indenture, will be deemed to include any such additional interest unless the context otherwise requires.

B-1

General. This Note is one of a duly authorized issue of Securities of the Company, issued and to be issued in one or more series under an indenture (the “Base Indenture”), dated as of February 26, 2008, between the Company and The Bank of New York Trust Company, N.A. (herein called the “Trustee,” which term includes any successor trustee under the Indenture with respect to a series of which this Note is a part), as supplemented by a First Supplemental Indenture thereto, dated as of March 4, 2008 (the “First Supplemental Indenture” and, together with the Base Indenture, the “Indenture”). Reference is hereby made to the Indenture for a statement of the respective rights, limitations of rights, duties and immunities thereunder of the Company, the Trustee and the Holders of the Securities, and of the terms upon which the Securities are, and are to be, authenticated and delivered. This Note is one of a duly authorized series of Securities designated as “6.875% Senior Notes due 2018” (collectively, the “Notes”), initially limited in aggregate principal amount to $550,000,000.

B-2

Indenture and all references to the Notes shall include the Additional Securities unless the context otherwise requires.

Sinking Fund. The Notes are not subject to any sinking fund.

B-3

Authorized Denominations. The Notes are issuable only in registered form without coupons in denominations of $2,000 or any integral multiple of $1,000 in excess thereof.

Defined Terms. All terms used in this Note, which are defined in the Indenture and are not otherwise defined herein, shall have the meanings assigned to them in the Indenture.

B- 4

B-5

IN WITNESS WHEREOF, the Company has caused this instrument to be duly executed and its seal to be hereunto affixed and attested.

Dated: March 4, 2008


	BIOGEN IDEC INC.
	By:
		Name:
		Title:

Attest:


By:
		Name:
		Title:

B-6

TRUSTEE’S CERTIFICATE OF AUTHENTICATION

This is one of the Notes of the series designated and referred to in the within-mentioned Indenture, as such is supplemented by the within-mentioned First Supplemental Indenture.


	THE BANK OF NEW YORK TRUST COMPANY, N.A., as Trustee
	By:
		Name:
		Title:

Dated: March 4, 2008

B-7

ASSIGNMENT

FOR VALUE RECEIVED, the undersigned hereby sell(s), assign(s) and transfer(s) unto

PLEASE INSERT SOCIAL SECURITY NUMBER OR OTHER IDENTIFYING NUMBER OF ASSIGNEE

(Please print or typewrite name and address,
including postal zip code, of assignee)

the within Note and all rights thereunder, and hereby irrevocably constitutes and appoints

to transfer said Note on the books of the Trustee, with full power of substitution in the premises.


Dated:

						NOTICE: The signature to this assignment must correspond with the name as written upon the face of the within Note in every particular, without alteration or enlargement or any change whatsoever.


Signature Guarantee

B-8

BIOGEN IDEC INC., as Issuer and The Bank of New York Trust Company, N.A., as Trustee First Supplemental Indenture Dated as of March 4, 2008

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