<PAGE>

                                                                    Exhibit 10.1

          Confidential Materials omitted and filed separately with the
         Securities and Exchange Commission. Asterisks denote omissions.

                       COLLABORATION AND LICENSE AGREEMENT

                                 BY AND BETWEEN

                          ALNYLAM PHARMACEUTICALS, INC.

                                       AND

                               BIOGEN IDEC MA INC.

Confidential

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                       COLLABORATION AND LICENSE AGREEMENT

This Collaboration and License Agreement dated the 20th day of September, 2006
(the "Execution Date") is by and between Alnylam Pharmaceuticals, Inc., a
corporation organized and existing under the laws of the State of Delaware and
having its principal office at 300 Third Street, Third Floor, Cambridge,
Massachusetts 02142 ("Alnylam"), and Biogen Idec MA Inc., a corporation
organized and existing under the laws of the Commonwealth of Massachusetts and
having its principal office at 14 Cambridge Center, Cambridge, MA 02142 ("Biogen
Idec").

                                  INTRODUCTION

WHEREAS, Alnylam has developed technology useful for the discovery, development,
manufacture, characterization or use of therapeutic products that function
through RNA interference;

WHEREAS, Biogen Idec is in the business of discovering, developing,
manufacturing and commercializing human therapeutic products;

WHEREAS, Alnylam and Biogen Idec are interested in collaborating, on the terms
and conditions set forth herein, in the discovery and development of RNAi
Therapeutics (as defined below) directed to the JC virus for use in the Field
(as defined below).

NOW, THEREFORE, Alnylam and Biogen Idec agree as follows:

                                    ARTICLE 1

                                   DEFINITIONS

When used in this Agreement, each of the following terms shall have the meanings
set forth in this Article 1:

     1.1  "ACTIVE IND" means an IND submitted to a Regulatory Authority by
          Biogen Idec or an Affiliate where such Regulatory Authority did not
          notify Biogen Idec or such Affiliate within thirty (30) days of such
          submission that it may not proceed under such IND.

     1.2  "AFFILIATE" means any corporation, company, partnership, joint venture
          and/or firm which controls, is controlled by, or is under common
          control with a specified person or entity. For purposes of this
          Section 1.2, "control" shall be presumed to exist if one of the
          following conditions is met: (a) in the case of corporate entities,
          direct or indirect ownership of at least fifty percent (50%) of the
          stock or shares having the right to vote for the election of
          directors, and (b) in the case of non-corporate entities, direct or
          indirect ownership of at least fifty percent (50%) of the equity
          interest with the power to direct the management and policies of such
          non-corporate entities. The Parties acknowledge that in the case of
          certain entities organized under the laws of certain countries outside
          of the United States, the maximum percentage ownership permitted by
          law for a foreign investor may

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          be less than fifty percent (50%), and that in such case such lower
          percentage shall be substituted in the preceding sentence, provided
          that such foreign investor has the power to direct the management and
          policies of such entity.

     1.3  "ALNYLAM COLLABORATION IP" means (a) any improvement, discovery or
          Know-How, patentable or otherwise, first identified, discovered or
          developed solely by employees of Alnylam or its Affiliates, or other
          persons not employed by Biogen Idec acting on behalf of Alnylam, under
          the Collaboration, and (b) any Patent Rights in the Territory which
          claim, cover or relate to such improvements, discoveries or Know-How
          and are Controlled by Alnylam at any time during the Collaboration
          Term. Alnylam Collaboration IP excludes Alnylam's interest in Joint
          Collaboration IP.

     1.4  "ALNYLAM IN-LICENSE" means an agreement between Alnylam and a Third
          Party pursuant to which Alnylam has rights and obligations with
          respect to, or which otherwise Cover, an RNAi Therapeutic and which is
          necessary to Discover, Develop, Commercialize and/or Manufacture an
          RNAi Therapeutic directed to JCV in the Field in the Territory,
          including without limitation the Existing Alnylam In-Licenses.

     1.5  "ALNYLAM KNOW-HOW" means Know-How that is either (a) Controlled by
          Alnylam on the Effective Date, or (b) comes within Alnylam's Control
          during the Term (other than Alnylam's rights in Joint Collaboration IP
          and Alnylam Collaboration IP).

     1.6  "ALNYLAM PATENT RIGHTS" means Patent Rights that (a) claim (i) Alnylam
          Know-How, or (ii) the identification, characterization, optimization,
          construction, expression, use or production of an RNAi Therapeutic,
          and which are useful or necessary to Discover, Develop, Commercialize
          and/or Manufacture a Selected RNAi Therapeutic in the Field in the
          Territory, and (b) are Controlled by Alnylam at any time during the
          Collaboration Term. As of the Execution Date, Alnylam Patent Rights
          include, without limitation, those listed on Schedule 1.6. Alnylam
          Patent Rights shall not include Patent Rights included in Alnylam
          Collaboration IP.

     1.7  "ALNYLAM TECHNOLOGY" means, collectively, Alnylam Know-How, Alnylam
          Patent Rights, Alnylam Collaboration IP and Alnylam's interest in
          Joint Collaboration IP.

     1.8  "ANNUAL WORK PLAN" means the master plan for a given Contract Year for
          the Discovery Program and the Development Program, an initial draft of
          which is set forth in Schedule 1.8 for the first Contract Year, as
          updated and approved by the Joint Steering Committee as necessary
          during each subsequent Contract Year.

     1.9  "BACK-UP CANDIDATE" means the RNAi Therapeutic directed to JCV that
          [**] and has been [**] as a back-up [**].


                                        3

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

     1.10 "BIOGEN IDEC COLLABORATION IP" means (a) any improvement, discovery or
          Know-How, patentable or otherwise, first identified, discovered or
          developed solely by employees of Biogen Idec or its Affiliates or
          other persons not employed by Alnylam acting on behalf of Biogen Idec,
          under the Collaboration, and (b) any Patent Rights in the Territory
          which claim, cover or relate to such improvements, discoveries or
          Know-How and are Controlled by Biogen Idec at any time during the
          Collaboration Term. Biogen Idec Collaboration IP excludes Biogen
          Idec's interest in Joint Collaboration IP.

     1.11 "BIOGEN IDEC IN-LICENSE" means an agreement between Biogen Idec and a
          Third Party pursuant to which Biogen Idec has rights and obligations
          with respect to, or which otherwise Cover, an RNAi Therapeutic and is
          necessary to Discover, Develop, Commercialize and/or Manufacture a
          Selected RNAi Therapeutic in the Field in the Territory. Biogen Idec
          In-Licenses as of the Execution Date are listed on Schedule 1.11.

     1.12 "BIOGEN IDEC KNOW-HOW" means Know-How that is either (a) Controlled by
          Biogen Idec on the Effective Date, or (b) comes within Biogen Idec's
          Control during the Collaboration Term (other than Biogen Idec's rights
          in Joint Collaboration IP and Biogen Idec Collaboration IP).

     1.13 "BIOGEN IDEC PATENT RIGHTS" means Patent Rights that (a) claim (i)
          Biogen Idec Know-How, or (ii) the identification, characterization,
          optimization, construction, expression, use or production of an RNAi
          Therapeutic, and which is useful or necessary to Discover, Develop,
          Commercialize and/or Manufacture RNAi Therapeutics in the Field in the
          Territory, and (b) are Controlled by Biogen Idec at any time during
          the Collaboration Term. Biogen Idec Patent Rights shall not include
          Patent Rights included in Biogen Idec Collaboration IP.

     1.14 "BIOGEN IDEC TECHNOLOGY" means, collectively, Biogen Idec Know-How,
          Biogen Idec Patent Rights, Biogen Idec Collaboration IP and Biogen
          Idec's interest in Joint Collaboration IP.

     1.15 "BLOCKING RNAi TECHNOLOGY" means Biogen Idec Collaboration IP
          comprising Broad RNAi Technology which would, in the absence of a
          license from Biogen Idec, be infringed by the practice of any
          invention claimed in a Valid Claim of the Alnylam Patent Rights
          existing at the end of the Discovery Term.

     1.16 "BROAD RNAi TECHNOLOGY" means any and all Know-How, whether or not
          patentable, that (a) is useful for the identification,
          characterization, optimization, construction, expression, use or
          production of an RNAi Therapeutic, (b) arises from or is necessary for
          the performance of the Collaboration and (c) which relates to
          structural features, properties, motifs, synthesis, analysis,
          modifications, delivery and/or uses of siRNAs that has broader utility
          beyond the specific embodiments represented by an RNAi Therapeutic
          directed to JCV and/or particular uses of such RNAi Therapeutic.


                                        4

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

     1.17 "BROAD RNAi TECHNOLOGY COLLABORATION IP" means Joint Collaboration IP
          that constitutes Broad RNAi Technology.

     1.18 "BUSINESS DAY" means a day on which banking institutions in Boston,
          Massachusetts are open for business.

     1.19 "CLINICAL PROOF OF CONCEPT" means, with respect to a Selected RNAi
          Therapeutic, either (a) [**] data from a clinical trial of such
          Selected RNAi Therapeutic [**]:

          (i)  [**]; or

          (ii) [**].

     1.20 "COLLABORATION" means the activities contemplated under this Agreement
          aimed at the Discovery of RNAi Therapeutics directed to JCV, and the
          Development and Manufacture of the Selected RNAi Therapeutics.

     1.21 "COLLABORATION TERM" means the period commencing on the Effective Date
          and ending on the earlier of (a) Regulatory Approval of a Selected
          RNAi Therapeutic anywhere in the Territory, and (b) the Established
          Utilization Date.

     1.22 "COMMERCIALIZATION" or "COMMERCIALIZE" means any activity to
          distribute, market, sell and/or have sold an approved Selected RNAi
          Therapeutic.

     1.23 "COMMERCIALLY REASONABLE EFFORTS" means the carrying out of
          obligations in a diligent and sustained manner using such effort and
          employing such resources (a) in Alnylam's case, as would normally be
          exerted or employed by a similarly situated biopharmaceutical company
          for a product of similar strategic importance, and at a similar stage
          of its product life, based on conditions then prevailing; and (b) in
          Biogen Idec's case, that are substantially similar to the effort and
          resources Biogen Idec would devote to a product of similar commercial
          or strategic importance, and at a similar stage of its product life,
          taking into consideration safety and efficacy, development costs, the
          anticipated prescription label, all payments due to Alnylam under this
          Agreement and all other relevant factors, taking into account the
          nature of the JCV RNAi Therapeutic and the clinical setting in which
          it is expected to be used. Commercially Reasonable Efforts shall be
          determined on a country-by-country basis.

     1.24 "CONFIDENTIAL INFORMATION" means any and all information and data,
          including without limitation all Alnylam Technology and Biogen Idec
          Technology, and all other scientific, pre-clinical, clinical,
          regulatory, manufacturing, marketing, financial and commercial
          information or data, whether communicated in writing or orally or by
          any other method, which is provided by one Party to the other Party in
          connection with this Agreement.


                                        5

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

     1.25 "CONTRACT YEAR" means the twelve (12) month period beginning on the
          Effective Date and each succeeding twelve (12) month period thereafter
          during the Term. Each Contract Year shall be divided into four (4)
          "Contract Quarters" comprised of successive three (3) month periods.

     1.26 "CONTROL" or "CONTROLLED" means with respect to any (a) material, item
          of information, method, data or other know-how, or (b) intellectual
          property right, the possession (whether by ownership or license, other
          than pursuant to this Agreement) by a Party or its Affiliates of the
          ability to grant to the other Party access and/or a license as
          provided herein under such item or right without violating the terms
          of any agreement or other arrangement with any Third Party existing
          before or after the Effective Date.

     1.27 "CORE JCV PATENTS" means Patent Rights Controlled by Alnylam, the
          claims of which are specifically directed to (a) RNAi Therapeutics in
          the Field, or (b) the manufacture or use of RNAi Therapeutics in the
          Field. As of the Effective Date, Core JCV Patents include those
          patents and applications set forth on Schedule 1.27.

     1.28 "COST OF GOODS" means, with respect to a Selected RNAi Therapeutic,
          the reasonable internal and external costs of Alnylam incurred in
          Manufacturing such Selected RNAi Therapeutic (including [**]),
          including: (a) to the extent that such Selected RNAi Therapeutic is
          sourced from Alnylam, the [**] cost of Manufacture of such Selected
          RNAi Therapeutic, consisting of [**], all calculated in accordance
          with generally accepted accounting principles in the United States
          consistently applied by Alnylam, and (b) to the extent that such
          Selected RNAi Therapeutic is sourced from a Third Party manufacturer,
          the actual price paid by Alnylam to the Third Party [**]. Cost of
          Goods shall not include [**].

     1.29 "COVERING", "COVER", or "COVERED" means, with respect to an RNAi
          Therapeutic, that in the absence of a license granted under a Valid
          Claim, the Discovery, Development, Manufacture or Commercialization of
          the RNAi Therapeutic would infringe such Valid Claim.

     1.30 "DEVELOPMENT" or "DEVELOP" means any preclinical and clinical activity
          to develop a Selected RNAi Therapeutic, beginning with IND-Enabling
          GLP Toxicology Studies and ending upon Regulatory Approval and
          completion of any post-approval regulatory commitments.

     1.31 "DEVELOPMENT CANDIDATE" means the RNAi Therapeutic directed to JCV
          that meets the Development Criteria and has been selected by Biogen
          Idec pursuant to Section 2.2 for further Development and
          Commercialization.

     1.32 "DEVELOPMENT CRITERIA" means the written criteria developed by the
          Parties and approved by the Joint Steering Committee, and included in
          the Annual Work Plan, for an RNAi Therapeutic directed to JCV to
          qualify as a Selected RNAi Therapeutic.


                                        6
<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

     1.33 "DEVELOPMENT MATERIALS" means animal models, cell lines, tissue
          samples, genes, plasmids, siRNAs, constructs, vectors, receptors and
          other proteins, peptides, and other biological materials related to
          RNAi Therapeutics that in each case are used in or that may be
          necessary or useful to conduct the Collaboration.

     1.34 "DISCOVERY" or "DISCOVER" means any and all activities to discover and
          characterize potential Development Candidates during the Discovery
          Term prior to the initiation of IND-Enabling Toxicology Studies,
          including without limitation, the research and development activities
          related to the generation, characterization, optimization,
          construction, synthesis, use and production of potential Development
          Candidates.

     1.35 "DISCOVERY TERM" means the period commencing on the Effective Date and
          continuing until the earlier to occur of (a) Alnylam having notified
          Biogen Idec of the Discovery of [**], or (b) the last date of the
          third Contract Year of the Term.

     1.36 "EFFECTIVE DATE" means September 20, 2006.

     1.37 "EMEA" means the European Medicines Agency and any successor agency
          having substantially the same functions.

     1.38 "ESTABLISHED UTILIZATION" means the [**].

     1.39 "ESTABLISHED UTILIZATION YEAR" means the twelve (12) month period
          beginning on the Established Utilization Date and each succeeding
          twelve (12) month period thereafter. Each Established Utilization Year
          shall be divided into four (4) "Established Utilization Quarters"
          comprised of successive three (3) month periods.

     1.40 "EXECUTIVE OFFICERS" means the Chief Executive Officer of Biogen Idec
          (or an executive of Biogen Idec designated by such Chief Executive
          Officer) and the Chief Executive Officer of Alnylam (or an executive
          of Alnylam designated by such Chief Executive Officer).

     1.41 "EXISTING ALNYLAM IN-LICENSES" means the Third Party agreements listed
          on Schedule 1.41, redacted copies of which have been provided to
          Biogen Idec prior to the Execution Date, as such schedule may be
          amended pursuant to Section 4.5.

     1.42 "FDA" means the United States Food and Drug Administration, or a
          successor agency thereto.

     1.43 "FIELD" means the prophylaxis and/or treatment of PML.

     1.44 "IND" means an Investigational New Drug application, Clinical Study
          Application, Clinical Trial Exemption, or similar application or
          submission for approval to conduct human clinical investigations filed
          with or submitted to a Regulatory Authority in conformance with the
          requirements of such Regulatory Authority.


                                        7

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

     1.45 "IND-ENABLING GLP TOXICOLOGY STUDIES" means genotoxicity, acute
          toxicology, safety pharmacology, and sub-chronic toxicology studies in
          species that satisfy applicable regulatory requirements using
          applicable good laboratory practices which meet the standard necessary
          for submission as part of an IND filing with a Regulatory Authority.

     1.46 "IN-LICENSES" means collectively, the Alnylam In-Licenses and the
          Biogen Idec In-Licenses.

     1.47 "JCV" means the human polyomavirus known as JC virus.

     1.48 "JCV RNAi THERAPEUTIC" means the [**].

     1.49 "JOINT COLLABORATION IP" means, collectively, (a) any improvement,
          discovery or Know-How, patentable or otherwise, first identified,
          discovered or developed jointly by the Parties or their Affiliates or
          others acting on behalf of Biogen Idec and Alnylam under the
          Collaboration, and (b) any Patent Rights in the Territory which claim,
          cover or relate to such improvements, discoveries or Know-How.

     1.50 "KNOW-HOW" means, with respect to an RNAi Therapeutic, all biological
          materials and other tangible materials, inventions, practices,
          methods, protocols, formulas, knowledge, know-how, trade secrets,
          processes, assays, skills, experience, techniques and results of
          experimentation and testing, including without limitation
          pharmacological, toxicological and pre-clinical and clinical test data
          and analytical and quality control data, patentable or otherwise,
          which relates to the identification, characterization, optimization,
          construction, expression, use or production of such RNAi Therapeutic
          and which are reasonably useful or necessary to Discover, Develop,
          Manufacture or Commercialize such RNAi Therapeutic in the Field in the
          Territory.

     1.51 "LINKED PRODUCT" means [**].

     1.52 "MAJOR MARKET" means any of the United States, the European Union,
          United Kingdom, France, Germany, Italy, Spain or Japan.

     1.53 "MANUFACTURING" or "MANUFACTURE" means, as applicable, all activities
          associated with the production, manufacture, processing, filling,
          finishing, packaging, labeling, shipping, and storage of RNAi
          Therapeutics, including process and formulation development, process
          validation, stability testing, manufacturing scale-up, pre-clinical,
          clinical and commercial manufacture and analytical development,
          product characterization, quality assurance and quality control,
          whether such activities are conducted by a Party, its Affiliates or a
          Third Party contractor of such Party.

     1.54 "NDA" means a New Drug Application, Biologics License Application,
          Worldwide Marketing Application, Marketing Authorization Application,
          Section 510(k) filing or similar application or submission filed with
          a Regulatory


                                        8

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          Authority in a country or group of countries to obtain marketing
          approval for a biological, pharmaceutical or other therapeutic product
          in that country or in that group of countries.

     1.55 "NET SALES" means, with respect to the Selected RNAi Therapeutics, the
          aggregate gross invoice prices of all units of such Selected RNAi
          Therapeutics sold by Biogen Idec and its Related Parties to Third
          Parties (other than a Sublicensee of Biogen Idec) after deducting, if
          not previously deducted, from the amount invoiced or received (a)
          trade and quantity discounts actually given other than early pay cash
          discounts; (b) returns, rebates, chargebacks and other allowances
          actually given; and (c) retroactive price reductions that are actually
          granted.

          Notwithstanding the foregoing, in the event a Selected RNAi
          Therapeutic is sold in conjunction with another clinically active
          therapeutic, prophylactic or diagnostic ingredient or component
          (whether packaged together or in the same formulation) (a "Combination
          Product"), Net Sales shall be calculated by multiplying the Net Sales
          of such Combination Product by a fraction, the numerator of which
          shall be the fair market value of the Selected RNAi Therapeutic
          included in such Combination Product as if sold separately (determined
          in accordance with generally accepted accounting principles), and the
          denominator of which shall be the aggregate fair market value of all
          the proprietary active ingredients of such Combination Product,
          including the Selected RNAi Therapeutic, as if sold separately. In the
          event no such separate sales are made by Biogen Idec or its Related
          Parties, Net Sales of the Combination Product shall be calculated in a
          manner to be negotiated and agreed upon by the Parties, reasonably and
          in good faith, prior to any sale of such Combination Product, which
          shall be based upon the respective estimated commercial values of the
          active components of such Combination Product.

          All references to Selected RNAi Therapeutics in this Agreement shall
          be deemed to include Combination Product, to the extent applicable.

          As provided in Section 5.4.3(b)(ii), Biogen Idec's or any of its
          Related Parties' transfer of Selected RNAi Therapeutics to another
          Related Party shall not result in any Net Sales, unless such Selected
          RNAi Therapeutic is consumed by such Related Party in the course of
          its commercial activities.

     1.56 "PARTY" or "PARTIES". "Party" means Biogen Idec or Alnylam; "Parties"
          means Biogen Idec and Alnylam.

     1.57 "PATENT RIGHTS" means all patents (including all reissues, extensions,
          substitutions, confirmations, re-registrations, re-examinations,
          invalidations, supplementary protection certificates and patents of
          addition) and patent applications (including all provisional
          applications, continuations, continuations-in-part and divisions).


                                        9

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

     1.58 "PML" means progressive multifocal leukoencephalopathy.

     1.59 "PRODUCT TRADEMARKS" means the trademark(s), service mark(s),
          accompanying logos, trade dress and/or indicia of origin used in
          connection with the distribution, marketing, promotion and sale of
          each Selected RNAi Therapeutic in the Territory. For purposes of
          clarity, the term Product Trademark(s) shall not include, without
          limitation, the corporate names and logos of either Party, and shall
          include any internet domain names incorporating such Product
          Trademarks.

     1.60 "REGULATORY APPROVAL" means the approval of the applicable Regulatory
          Authority necessary for the marketing and sale of a Selected RNAi
          Therapeutic in a particular indication in the Field in a country,
          including, where required, separate pricing and/or reimbursement
          approvals.

     1.61 "REGULATORY AUTHORITY" means any applicable government regulatory
          authority involved in granting approvals for the Development,
          Manufacturing, or Commercialization of a Selected RNAi Therapeutic in
          the Territory, including without limitation the FDA and the EMEA.

     1.62 "RELATED PARTY" means Biogen Idec's Affiliates and permitted
          Sublicensees, which term does not include wholesale distributors of
          Biogen Idec or its Affiliates, who purchase a Selected RNAi
          Therapeutic from Biogen Idec or its Affiliates in an arm's length
          transaction and who have no other obligation, including without
          limitation a reporting obligation, to Biogen Idec or its Affiliates.

     1.63 "RNAi THERAPEUTIC" means a therapeutic product containing, comprised
          of or based on one or more siRNAs or siRNA derivatives or other
          moieties effective in gene function modulation and designed to
          modulate the function of particular genes or gene products through RNA
          interference.

     1.64 "SELECTED RNAi THERAPEUTIC" means the [**].

     1.65 "SELECTED RNAi THERAPEUTICS" means, collectively, [**].

     1.66 "siRNA" means a double-stranded ribonucleic acid (RNA) composition
          designed to act primarily through an RNA interference mechanism that
          consists of either (a) two separate oligomers of native or chemically
          modified RNA that are hybridized to one another along a substantial
          portion of their lengths, or (b) a single oligomer of native or
          chemically modified RNA that is hybridized to itself by
          self-complementary base-pairing along a substantial portion of its
          length to form a hairpin.

     1.67 "SUBLICENSEE" means a Third Party to whom a Party grants a sublicense
          under any Alnylam Technology (in the case of Biogen Idec), Biogen Idec
          Technology (in the case of Alnylam) or Joint Collaboration IP (in the
          case of either Party) as permitted under this Agreement.


                                       10
<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

     1.68 "TERRITORY" means all the countries of the world.

     1.69 "THIRD PARTY" means any person or entity other than a Party or any of
          its Affiliates.

     1.70 "VALID CLAIM" means a claim of: (a) an issued and unexpired Patent
          Right, which claim has not been revoked or held unenforceable,
          unpatentable or invalid by a decision of a court or other governmental
          agency of competent jurisdiction, which is not appealable or has not
          been appealed within the time allowed for appeal, and which has not
          been abandoned, disclaimed, denied or admitted to be invalid or
          unenforceable through reissue, re-examination or disclaimer or
          otherwise, or (b) a patent application for a patent included within
          the Patent Rights that has been pending less than [**] years from the
          earliest date on which such patent application claims priority and
          which claim has not been cancelled, withdrawn or abandoned or finally
          rejected by an administrative agency action from which no appeal can
          be taken.

     1.71 ADDITIONAL DEFINITIONS. The following terms have the meanings set
          forth in the corresponding Sections of this Agreement:

<TABLE>
<CAPTION>
TERM                                   SECTION
----                                   -------
<S>                                    <C>
"AGREEMENT TERM"                       8.1
"ALNYLAM INDEMNITEES"                  9.5.1
"BANKRUPT PARTY"                       8.2.2(c)
"BIOGEN IDEC INDEMNITEES"              9.5.2
"BREACHING PARTY"                      8.2.1
"CODE"                                 8.2.2(c)
"COLLABORATION MANAGER"                3.2
"COMBINATION PRODUCT"                  1.53
"[**] SUBLICENSE AGREEMENT"            4.5
"DEFENDING PARTY"                      6.5.3
"DEVELOPMENT PROGRAM"                  2.3
"DISCOVERY PROGRAM"                    2.2
"ELAN"                                 10.2.2
"ESTABLISHED UTILIZATION DATE"         5.3
"FTE"                                  5.1.1
"FTE RATE"                             5.1.1
"GRANTING PARTY"                       4.1.5
"INDEMNITEE"                           9.5.4
"INFRINGEMENT CLAIM"                   6.5.1
"INITIAL ENFORCEMENT RIGHTS PARTY"     6.4.3
"JSC" OR "JOINT STEERING COMMITTEE"    3.1
"JSC CHAIRPERSON"                      3.1.2
"LOSSES"                               9.5.1
"NON-BANKRUPT PARTY"                   8.2.2
"PATENT EXPENSES"                      6.3.7
"PROSECUTING PARTY"                    6.3.5
</TABLE>


                                       11

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

<TABLE>
<S>                                    <C>
"SECONDARY ENFORCEMENT RIGHTS PARTY"   6.4.3
"SPC"                                  6.8
"STANFORD AGREEMENT"                   4.5
"THIRD PARTY COLLABORATION"            2.6.2
"THIRD PARTY PERFORMERS"               2.6
</TABLE>

                                    ARTICLE 2

                                  COLLABORATION

     2.1  OVERVIEW. The Parties shall collaborate in carrying out the
          Collaboration with the objective of Discovering and Developing RNAi
          Therapeutics directed to JCV in the Field for further
          Commercialization by Biogen Idec. Alnylam shall be responsible for the
          Discovery of such RNAi Therapeutics, and will present Biogen Idec with
          up to [**] RNAi Therapeutics directed to JCV that meet the Development
          Criteria and from which Biogen Idec shall select [**] for further
          Development under the Collaboration. Subject to the terms of this
          Agreement, Biogen Idec will be responsible for the Development of [**]
          Selected RNAi Therapeutics and for the further Commercialization of
          such Selected RNAi Therapeutics. The Discovery of such RNAi
          Therapeutics and the Development of the Selected RNAi Therapeutics
          shall be governed by the Annual Work Plan, and the Parties agree to
          conduct all their Discovery and Development activities in accordance
          with the Annual Work Plan. For the avoidance of doubt, Biogen Idec
          shall have the right to Develop and Commercialize [**], but shall have
          no obligation to Develop and Commercialize [**].

     2.2  DISCOVERY PROGRAM. During the Discovery Term, Alnylam shall use
          Commercially Reasonable Efforts to Discover [**] RNAi Therapeutics
          directed to JCV that meet the Development Criteria agreed upon by the
          JSC and included in the Annual Work Plan (the "Discovery Program").
          Alnylam shall from time to time notify the JSC in writing that it
          believes that a particular RNAi Therapeutic directed to JCV Discovered
          by Alnylam meets the Development Criteria. Upon such notice, the JSC
          will promptly meet and Alnylam shall provide the JSC with the data and
          information in its possession and Control regarding the applicable
          RNAi Therapeutic. Alnylam shall present Biogen Idec with [**] RNAi
          Therapeutics in accordance with this Section 2.2. Biogen Idec shall
          have the right to select [**] RNAi Therapeutics presented by Alnylam
          by written notice to Alnylam. Biogen Idec may make its selection [**]
          at any time after Alnylam's presentation of the first RNAi Therapeutic
          to the JSC, but Biogen Idec must identify [**] no later than [**] days
          after Alnylam's presentation of [**] to the JSC. For avoidance of
          doubt, if Biogen Idec selects [**] prior to Alnylam's presentation of
          [**] to the JSC, Alnylam will not be relieved of its obligation to
          present [**] to the JSC. In the event that Biogen Idec wishes to [**],
          Biogen Idec shall do so by written notice to Alnylam no later than
          [**] days after Alnylam's presentation to the JSC of [**].

     2.3  DEVELOPMENT PROGRAM. Each Party shall use Commercially Reasonable
          Efforts to fulfill its obligations to Develop at least one Selected
          RNAi Therapeutic as set forth in the Annual Work Plan. Except as
          otherwise agreed by the JSC in writing, all activities


                                       12

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          relating to the Development of each Selected RNAi Therapeutic (the
          "Development Program") will be set forth in the Annual Work Plan.

     2.4  ANNUAL WORK PLAN. The Parties shall prepare and submit for approval by
          the JSC the Annual Work Plan for every Contract Year at least [**]
          days prior to the commencement of such Contract Year or, in the case
          of the first Contract Year only, within [**] days after the Effective
          Date. The first Annual Work Plan shall be based on and consistent with
          the draft Annual Work Plan attached as Schedule 1.8. The JSC shall
          update and amend, as appropriate, the then-current Annual Work Plan
          from time to time. All such updates or amendments shall be subject to
          approval of the JSC. Each Annual Work Plan shall contain the specific
          objectives to be achieved during the Contract Year, the specific
          activities to be performed under the Discovery Program and the
          Development Program, the timeline for performing such activities and
          the Development Criteria. The Annual Work Plan shall allocate
          Development tasks between the Parties consistent with their respective
          capabilities and with the respective responsibilities of the Parties
          as set forth in Sections 2.9 and 2.10 below, and to the extent
          possible and scientifically sound, in a manner to maximize the
          expeditious and cost-effective Development and Manufacture of each
          Selected RNAi Therapeutic. Each Annual Work Plan shall be accompanied
          by a budget prepared by the JSC setting forth the projected internal
          and external resources and expenses for the Discovery Program and the
          Development Program, including quarterly cost detail for the upcoming
          calendar year, quarterly forecast of Net Sales, if applicable, for the
          upcoming calendar year, and the number of Alnylam FTE's expected to be
          required. In addition, the JSC shall approve a long-range budget
          covering activities anticipated under Annual Work Plans for each of
          the subsequent [**] years. In recognition of the importance of
          budgets, each Party shall have the full opportunity to review and
          comment on budgets before final approval. The JSC will update the
          budget as necessary to reflect approved changes in the Parties'
          activities or changes in expected costs for those activities. Such
          budget shall be reviewed and approved at least annually by the JSC
          pursuant to Section 3.1.4(c).

     2.5  [**]. Biogen Idec may at any time, by written notice to the JSC, elect
          to [**] the JCV RNAi Therapeutic, and the Parties shall promptly
          develop a revised Annual Work Plan to be approved by the JSC.
          Effective upon such notice (a) all references in this Agreement to
          [**] will automatically be deemed to refer to [**], and (b) all
          references to [**] will be deemed to refer to [**].

     2.6  THIRD PARTIES.

          2.6.1 In General. The Parties shall be entitled to utilize the
               services of Third Parties, including Third Party contract
               research organizations and service providers, and to collaborate
               with Third Parties ("Third Party Performers"), to perform their
               respective Discovery, Development and Manufacturing activities;
               provided that each Party shall remain at all times fully liable
               for its respective responsibilities under this Agreement and the
               Annual Work Plan. Except as expressly permitted under Section
               2.6.2, neither Party shall use Third Party Performers to conduct
               part or all of its obligations under this Agreement or the
               Collaboration unless the contracting or collaborating party's
               agreement with the Third Party Performer


                                       13

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

               provides the non-contracting or non-collaborating Party with the
               same rights under this Agreement as if the contracting or
               collaborating Party had done the work itself, and any such Third
               Party agreement shall include confidentiality and non-use
               provisions which are no less stringent than those set forth in
               Article 7 of this Agreement.

          2.6.2 Third Party Collaborations. In the event that, despite good
               faith efforts, the contracting Party is unable to enter into an
               agreement with a Third Party Performer that meets the
               requirements of the second sentence of Section 2.6.1, such
               activities will be termed a "Third Party Collaboration." If any
               Third Party Collaborations are contemplated in connection with
               the Collaboration, such Third Party Collaborations will be
               approved in advance by the JSC, and the Annual Work Plan shall be
               amended to include such approved Third Party Collaborations. The
               Parties shall use good faith efforts to ensure that, to the
               extent possible, all such Third Party Collaborations shall
               provide that any and all data and results, discoveries and
               inventions, whether patentable or not, arising out of the Third
               Party Collaboration may be used by bona fide collaborators of the
               Party entering into the Third Party Collaboration agreement. In
               addition, a Party entering into any such Third Party
               Collaboration shall use reasonable efforts to obtain a right to
               sublicense to the other Party and its Affiliates and Sublicensees
               any intellectual property arising out of the Third Party
               Collaboration to the extent necessary for the other Party to
               perform its obligations under this Agreement. Except as expressly
               stated in this Section 2.6.2, all Third Party Collaborations must
               meet the requirements of Section 2.6.1.

     2.7  RECORDS. Each Party shall maintain scientific records, in sufficient
          detail and in good scientific manner appropriate for patent and
          regulatory purposes, which shall fully and properly reflect all work
          done and results achieved in the performance of the Collaboration by
          such Party. Each Party shall have the right, during normal business
          hours and upon reasonable notice, to inspect and copy (or request the
          other Party to copy) all records of the other Party maintained in
          connection with the work done and results achieved in the performance
          of the Collaboration, but solely to the extent access to such records
          is necessary for a Party to exercise its rights under this Agreement.
          All such records and the information disclosed therein shall be
          maintained in confidence by the recipient in accordance with Article
          7.

     2.8  DEVELOPMENT MATERIAL TRANSFER. In order to facilitate the
          Collaboration, either Party may provide to the other Party certain
          Development Materials Controlled by the supplying Party (other than
          under this Agreement) for use by the other Party in furtherance of the
          Collaboration. Except as otherwise provided under this Agreement or
          explicitly authorized in writing by the supplying Party, all such
          Development Materials delivered to the other Party shall remain the
          sole property of the supplying Party, shall be used only in
          furtherance of the Collaboration and solely under the control of the
          other Party and its Affiliates, shall not be used or delivered to or
          for the benefit of any Third Party without the prior written consent
          of the supplying Party, and shall not be used in research or testing
          involving human subjects. The Development Materials supplied under


                                       14

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          this Section 2.8 must be used with prudence and appropriate caution in
          any experimental work, since not all of their characteristics may be
          known. THE DEVELOPMENT MATERIALS ARE PROVIDED "AS IS" AND WITHOUT ANY
          REPRESENTATION OR WARRANTY, EXPRESS OR IMPLIED, INCLUDING WITHOUT
          LIMITATION ANY IMPLIED WARRANTY OF MERCHANTABILITY OR OF FITNESS FOR
          ANY PARTICULAR PURPOSE OR, SAVE AS EXPLICITLY PROVIDED IN THIS
          AGREEMENT, ANY WARRANTY THAT THE USE OF THE MATERIALS WILL NOT
          INFRINGE OR VIOLATE ANY PATENT OR OTHER PROPRIETARY RIGHTS OF ANY
          THIRD PARTY.

     2.9  REGULATORY MATTERS.

          2.9.1 Coordination. Biogen Idec shall oversee, monitor, coordinate,
               file and hold in its name all regulatory actions, communications
               and filings with and submissions, including filings and
               submissions of supplements and amendments thereto, to the
               applicable Regulatory Authorities with respect to the Selected
               RNAi Therapeutics; provided, however, that Biogen Idec shall
               consult with Alnylam regarding [**].

          2.9.2 Regulatory Meetings and Correspondence. Biogen Idec shall be
               responsible for interfacing, corresponding and meeting with the
               applicable Regulatory Authorities with respect to the Selected
               RNAi Therapeutics. Upon the request of Biogen Idec, Alnylam shall
               [**] the applicable Regulatory Authorities with respect to each
               Selected RNAi Therapeutic.

          2.9.3 Review of Correspondence. To the extent practicable, Biogen Idec
               shall provide Alnylam with [**] the applicable Regulatory
               Authorities pertaining to each Selected RNAi Therapeutic, [**],
               and, [**] Biogen Idec, Alnylam shall [**]. Biogen Idec shall
               provide to Alnylam as soon as reasonably practicable [**] such
               Regulatory Authorities pertaining to each Selected RNAi
               Therapeutic (including without limitation [**]).

          2.9.4 Assistance. Each Party shall cooperate with the other Party to
               provide all reasonable assistance and take all actions reasonably
               requested by the other Party that are necessary or desirable to
               enable the other Party to comply with any law or regulation
               applicable to the Selected RNAi Therapeutics or the
               Collaboration, including, but not limited to, reporting adverse
               drug experiences (and serious adverse drug experiences) to the
               applicable Regulatory Authorities.

     2.10 MANUFACTURE AND SUPPLY.

          2.10.1 Pre-Clinical and Clinical Supply. During the Collaboration
               Term, Alnylam will use Commercially Reasonable Efforts to
               Manufacture and supply the Parties' requirements of the Selected
               RNAi Therapeutics for pre-clinical and clinical Discovery and
               Development purposes, including those requirements for
               manufacture and supply of Selected RNAi Therapeutics for
               administration outside of formal clinical trials in the manner
               described in Section 1.38(a). Biogen Idec


                                       15

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

               will pay Alnylam for the quantities of Selected RNAi Therapeutics
               supplied by Alnylam hereunder in accordance with Section 5.1.2.
               Within [**] after the Effective Date, the Parties shall enter
               into a supply agreement reasonably acceptable to both Parties
               pursuant to which Alnylam will supply the Selected RNAi
               Therapeutics to Biogen Idec. The aforementioned supply agreement
               shall provide that (a) [**] Manufacturing each Selected RNAi
               Therapeutic [**]; (b) any such [**] will be subject to [**]; and
               (c) Biogen Idec shall [**] Selected RNAi Therapeutics for
               pre-clinical and clinical Discovery and Development. Such supply
               agreement will also explicitly provide that [**].

          2.10.2 Commercial Supply. Biogen Idec shall be responsible for the
               Manufacture and supply of the Selected RNAi Therapeutics after
               the Collaboration Term and for commercial sale throughout the
               Territory. Alnylam shall provide [**]; provided that (a) any such
               [**]; and (b) [**] of Alnylam.

     2.11 COMMERCIALIZATION. Subject to the terms of this Agreement, Biogen Idec
          shall be solely responsible for all Commercialization activities
          relating to the Selected RNAi Therapeutics and shall bear one hundred
          percent (100%) of all expenses for the Commercialization of such
          Selected RNAi Therapeutics. Biogen Idec shall use Commercially
          Reasonable Efforts to Commercialize the JCV RNAi Therapeutic in the
          Major Markets.

                                    ARTICLE 3

                            COLLABORATION MANAGEMENT

     3.1  JOINT STEERING COMMITTEE. As soon as practicable after the Effective
          Date, the Parties shall establish a committee to facilitate the
          Collaboration (the "Joint Steering Committee" or "JSC") as follows:

          3.1.1 Composition of the JSC. The Collaboration shall be conducted
               under the direction of a JSC comprised of three (3)
               representatives of Alnylam and three (3) representatives of
               Biogen Idec. Each Party shall appoint its respective
               representatives to the JSC from time to time, and may substitute
               one or more of its representatives, in its sole discretion,
               effective upon notice to the other Party of such change. Each
               Party shall have at least one JSC representative who is a senior
               employee (director level or above), and all JSC representatives
               shall have appropriate research, preclinical, manufacturing,
               clinical development or commercialization expertise and ongoing
               familiarity with the Collaboration. Additional representatives or
               consultants may from time to time, by mutual consent of the
               Parties, be invited to attend JSC meetings, subject to such
               representatives' and consultants' written agreement to comply
               with the requirements of Article 7. Each Party shall bear its own
               expenses relating to attendance at such meetings by its
               representatives.


                                       16

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          3.1.2 JSC Chairperson. The "JSC Chairperson" shall be a representative
               of Biogen Idec. The JSC Chairperson's responsibilities shall
               include (a) scheduling meetings at least once per Contract
               Quarter, but more frequently if the JSC determines it necessary;
               (b) setting agendas for meetings with solicited input from other
               members; (c) confirming and delivering minutes to the JSC for
               review and final approval; and (d) conducting effective meetings,
               including ensuring that objectives for each meeting are set and
               achieved.

          3.1.3 Meetings. The JSC shall meet in accordance with a schedule
               established by mutual written agreement of the Parties, but no
               less frequently than once per Contract Quarter, with the location
               for such meetings alternating between Alnylam and Biogen Idec
               facilities (or such other locations as are determined by the
               JSC). Alternatively, the JSC may meet by means of teleconference,
               videoconference or other similar communications equipment, but at
               least two meetings per year shall be conducted in person.

          3.1.4 JSC Responsibilities. The JSC shall have the following
               responsibilities with respect to the Collaboration:

               (a)  determining the overall Discovery and Development strategy
                    for each Selected RNAi Therapeutic in the Territory;

               (b)  reviewing for approval the initial Annual Work Plan
                    (including without limitation the Development Criteria)
                    within [**] days after the Effective Date, with appropriate
                    input from Alnylam and Biogen Idec senior management;

               (c)  reviewing for approval (i) the annual update to the Annual
                    Work Plan and the related budget described in Section 2.4,
                    and (ii) any modifications to such Annual Work Plan and
                    related budget, in each case within [**] days of each
                    submission to the JSC;

               (d)  determining each Party's responsibilities under the Annual
                    Work Plan consistent with Section 2.4; provided, however,
                    that the JSC may not, without Alnylam's consent, assign
                    Alnylam any responsibilities under the Annual Work Plan for
                    which Alnylam does not at the time already have sufficient
                    internal expertise and resources;

               (e)  facilitating the transfer of Know-How and Confidential
                    Information between the Parties for purposes of conducting
                    the Annual Work Plan;

               (f)  regularly assessing the progress of the Parties in their
                    conduct of the Annual Work Plan and against the timelines
                    and budgets contained therein, reviewing relevant data, and
                    considering issues of priority;

               (g)  reviewing for approval, within the Parties' regular business
                    cycles, but no later than [**] days after a proposal is made
                    to the JSC by either Party, the


                                       17

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                    initiation of IND-Enabling Toxicology Studies and the
                    submission of an IND with respect to each Selected RNAi
                    Therapeutic; and

               (h)  performing such other activities as are contemplated under
                    this Agreement and that the Parties mutually agree shall be
                    the responsibility of the JSC.

     3.2  APPOINTMENT OF SUBCOMMITTEES, PROJECT TEAMS AND COLLABORATION
          MANAGERS. The JSC shall be empowered to create such subcommittees of
          itself and additional project teams as it may deem appropriate or
          necessary. Each such subcommittee and project team shall report to the
          JSC, which shall have authority to approve or reject recommendations
          or actions proposed thereby subject to the terms of this Agreement.
          Each Party shall also designate a "Collaboration Manager." The
          Collaboration Managers will be responsible for the day-to-day
          coordination of the Collaboration and will serve to facilitate
          communication between the Parties. Each Party may change its
          designated Collaboration Manager from time to time upon written notice
          to the other Party.

     3.3  REPORTS AND MINUTES. Each Party will provide the members of the JSC
          with written copies of all materials they intend to present at the JSC
          meeting. The JSC may also request at any time specific data or
          information related to Development activities or that a written report
          be prepared in advance of any meeting summarizing certain material
          data and information arising out of the conduct of the Development
          activities and the Party or appropriate committee to whom such request
          is made shall promptly provide to the other Party or JSC such report,
          data or information. A secretary shall be appointed for each meeting
          and shall prepare minutes of the meeting, which shall provide a
          description in reasonable detail of the discussions held at the
          meeting and a list of any actions, decisions or determinations
          approved by the JSC.

     3.4  DECISION-MAKING AND DISPUTE RESOLUTION.

          3.4.1 Voting. With respect to decisions of the JSC, the
               representatives of each Party shall have collectively one vote on
               behalf of such Party. For each meeting of the JSC, at least two
               (2) representatives of each Party shall constitute a quorum.
               Action on any matter may be taken at a meeting, by
               teleconference, videoconference or by written agreement.

          3.4.2 Decision-Making. The JSC shall operate by consensus, subject to
               the dispute resolution process set forth in Section 3.4.3 below.

          3.4.3 Dispute Resolution. The JSC shall attempt to resolve any and all
               disputes relating to the Collaboration by unanimous consensus. In
               the event the JSC is unable to reach a unanimous consensus with
               respect to any such dispute, then the following dispute
               resolution provisions shall apply.

               (a)  With respect to any dispute over which [**] except those
                    disputes related to [**]; provided, however, that [**] shall
                    have the right to propose [**], for good faith consideration
                    by [**], and if [**], then [**] shall [**]. Notwithstanding
                    anything to the contrary, [**] may not, without


                                       18

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                    Alnylam's consent, increase Alnylam's obligation to provide
                    FTE's in performance of Alnylam's activities under the
                    Annual Work Plan by more than [**] percent ([**]%) of the
                    number of FTE's required in the preceding year.

               (b)  Notwithstanding anything else contained herein,
                    Commercialization activities shall not be the responsibility
                    of the JSC and shall be subject to Biogen Idec's final
                    decision-making authority.

               (c)  With respect to any dispute concerning [**] the dispute
                    shall be [**] as described in this paragraph. If the dispute
                    cannot be resolved [**], then within [**] days after
                    [**]shall be [**] in good faith as soon as is practicable
                    but in no event [**]. Such resolution, if any, by [**] shall
                    be final and binding on the Parties. If the [**] shall have
                    [**] decision-making authority.

               (d)  With respect to all other disputes between the Parties
                    regarding the interpretation, construction or application of
                    this Agreement, the dispute shall be [**] for review as
                    described in this paragraph. If the dispute cannot be
                    resolved [**], then within [**] days after [**] shall be
                    [**] as soon as is practicable but in no event [**]. Such
                    resolution, if any, by [**] shall be final and binding on
                    the Parties. If the [**], each Party will be free to pursue
                    all rights available to it under law or equity.

     3.5  DISSOLUTION OF JSC. The JSC shall be dissolved at the end of the
          Collaboration Term; provided that Alnylam's participation in the JSC
          after the fifth (5th) anniversary of the Effective Date will be at
          Alnylam's sole option.

                                    ARTICLE 4

                                    LICENSES

     4.1  LICENSE GRANTS. The following Licenses are granted as of the Effective
          Date:

          4.1.1 Discovery License. Subject to the terms and conditions of this
               Agreement, Biogen Idec hereby grants Alnylam a co-exclusive (with
               Biogen Idec) royalty-free license under Biogen Idec Technology to
               Discover RNAi Therapeutics directed to JCV and to otherwise
               conduct the Discovery Program in the Field, in each case to the
               extent necessary for Alnylam to conduct its activities under the
               Annual Work Plan. Subject to the terms and conditions of this
               Agreement, Alnylam hereby grants Biogen Idec a co-exclusive (with
               Alnylam), royalty-free license under Alnylam Technology to
               Discover RNAi Therapeutics directed to JCV and to otherwise
               conduct the Discovery Program in the Field, in each case to the
               extent necessary for Biogen Idec to conduct its activities under
               the Annual Work Plan.


                                       19

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          4.1.2 Development Licenses. Subject to the terms and conditions of
               this Agreement, Alnylam hereby grants Biogen Idec a co-exclusive
               (with Alnylam), royalty-free license under Alnylam Technology to
               Develop the Selected RNAi Therapeutics in the Field in the
               Territory, to the extent necessary for Biogen Idec to conduct its
               activities under the Annual Work Plan. Subject to the terms and
               conditions of this Agreement, Biogen Idec hereby grants Alnylam a
               co-exclusive (with Biogen Idec), royalty-free license under
               Biogen Idec Technology to Develop the Selected RNAi Therapeutics
               in the Field in the Territory, to the extent necessary for
               Alnylam to conduct its activities under the Annual Work Plan.

          4.1.3 Commercialization License. Subject to the terms and conditions
               of this Agreement, Alnylam hereby grants Biogen Idec an
               exclusive, royalty-bearing license under Alnylam Technology to
               Commercialize the Selected RNAi Therapeutics in the Field in the
               Territory.

          4.1.4 Manufacturing Licenses. Subject to the terms and conditions of
               this Agreement, only as permitted and solely for the purposes set
               forth in Section 2.10.2, Alnylam hereby grants Biogen Idec a
               non-exclusive, fully-paid, royalty-free license under Alnylam
               Technology to Manufacture the Selected RNAi Therapeutics for the
               Territory. Subject to the terms and conditions of this Agreement,
               only as permitted and solely for the purposes set forth in
               Section 2.10.1, Biogen Idec hereby grants Alnylam a non-exclusive
               license under Biogen Idec Technology to Manufacture the Selected
               RNAi Therapeutics.

          4.1.5 Blocking RNAi Technology License. Biogen Idec hereby grants
               Alnylam a non-exclusive, royalty-free license, with the right to
               grant sublicenses, under any Blocking RNAi Technology for the
               purpose of research, development, manufacture, use, import,
               distribution, marketing or sale of RNAi Therapeutics outside the
               Field.

     4.2  SUBLICENSES AND LICENSES OF JOINT COLLABORATION IP. The licenses in
          Section 4.1 each includes the right to grant sublicenses and licenses
          as provided in this Section 4.2 below.

          4.2.1 Affiliates. Each Party shall be entitled to grant sublicenses of
               its rights under this Agreement (and licenses under any Joint
               Collaboration IP) to any of its Affiliates for so long as such
               entity remains an Affiliate.

          4.2.2 Academic Collaborators and Service Companies. Each Party shall
               be entitled to grant sublicenses of its rights under Sections
               4.1.1 and 4.1.2 (and licenses under any Joint Collaboration IP)
               in connection with its activities under the Annual Work Plan to
               academic collaborators, and contract service organizations, that
               in each case are subject to and fulfill the requirements of
               Section 2.6, to perform its obligations under such Annual Work
               Plan.

          4.2.3 Manufacturing Sublicenses. Subject to the provisions of Sections
               2.10.1 and 2.10.2 regarding approval of subcontractors, each
               Party entitled to Manufacture RNAi Therapeutics under the terms
               and conditions of this Agreement shall be


                                       20

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

               entitled to grant sublicenses of its rights under this Agreement
               (and licenses under any Joint Collaboration IP) to Third Parties
               to Manufacture such RNAi Therapeutics; provided, that such Party
               shall remain primarily responsible with respect to such
               sublicense.

          4.2.4 Terms. Each sublicense granted by a Party pursuant to Section
               4.2.2 or Section 4.2.3 shall be subject and subordinate to the
               terms and conditions of this Agreement and shall contain terms
               and conditions consistent with those in this Agreement.
               Agreements with any Commercializing Sublicensee shall contain the
               following provisions: (a) a requirement that such Sublicensee
               submit applicable sales or other reports consistent with those
               required hereunder; (b) an audit requirement similar to the
               requirement set forth in Section 5.5; and (c) a requirement that
               such Sublicensee comply with the confidentiality and non-use
               provisions of Article 7 with respect to both Parties'
               Confidential Information.

          4.2.5 Liability. Each Party shall at all times be responsible for the
               performance of its Sublicensees under this Agreement.

          4.2.6 Sublicense Survival. Any sublicense by a Party shall be subject
               to the provisions of this Agreement and shall survive termination
               of the licenses or other rights granted to such Party under this
               Agreement in accordance with this Section 4 and be assumed by the
               other Party as long as (a) the Sublicensee is not then in breach
               of its sublicense agreement, (b) the Sublicensee agrees in
               writing to be bound to the non-sublicensing Party as a licensor
               under the terms and conditions of the sublicense agreement, and
               (iii) the Sublicensee agrees in writing that in no event shall
               the non-sublicensing Party assume any obligations or liabilities,
               or be under any obligation or requirement of performance, under
               any such sublicense extending beyond such Party's obligations and
               liabilities under this Agreement.

     4.3  JOINT COLLABORATION IP. Subject to the rights granted each Party under
          this Agreement, each Party shall have the right to use, sell, keep,
          license or assign its interest in Joint Collaboration IP and otherwise
          undertake all activities a sole owner might undertake with respect to
          such Joint Collaboration IP without the consent of and without
          accounting to the other Party.

     4.4  IN-LICENSES. All licenses and other rights granted Biogen Idec under
          this Article 4 are subject to the rights and obligations of Alnylam
          under the Alnylam In-Licenses. All licenses and other rights granted
          Alnylam under this Article 4 are subject to the rights and obligations
          of Biogen Idec under the Biogen Idec In-Licenses.

     4.5  CERTAIN PATENT RIGHTS. Notwithstanding anything to the contrary
          herein, the licenses to Alnylam Patent Rights hereunder initially
          shall not include licenses to Patent Rights licensed by Alnylam or its
          Affiliates under the [**]; provided that if any such Patent Rights
          in-licensed by Alnylam become issued Patent Rights that Cover a
          Selected RNAi Therapeutic, Alnylam shall so notify Biogen Idec in
          writing, and Biogen Idec shall have the option of expanding its
          licenses to Alnylam Patent Rights hereunder to include such issued
          Patent Rights by notifying Alnylam of such election in writing. Upon
          such


                                       21

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          election, the [**] Agreement shall be deemed an Existing Alnylam
          In-License and Schedule 1.41 shall be amended accordingly without any
          further consideration due from Biogen Idec to Alnylam. Further,
          notwithstanding anything to the contrary herein, the licenses to
          Alnylam Patent Rights hereunder shall not include licenses to Patent
          Rights licensed by Alnylam or its Affiliates under the License
          Agreement between [**]. The Parties shall simultaneously with the
          execution of this Agreement enter into an agreement substantially in
          the form set forth in Schedule 4.5 (the "[**] Sublicense Agreement").

     4.6  TERMINATION OF CERTAIN LICENSE RIGHTS. Notwithstanding anything to the
          contrary in this Agreement, in the event that Biogen Idec imposes any
          conditions of access to or treatment with Selected RNAi Therapeutic by
          patients who have been treated using products from Third Parties that
          compete directly with Linked Products, then all rights and licenses
          granted to Biogen Idec with respect to [**], whichever is less
          advanced in the Development and Commercialization process, shall
          terminate and revert to Alnylam.

     4.7  NO OTHER RIGHTS. Except as otherwise expressly provided in this
          Agreement, under no circumstances shall a Party hereto, as a result of
          this Agreement, obtain any ownership interest or other right in any
          Know-How or Patent Rights of the other Party, including items owned,
          controlled or developed by the other Party, or provided by the other
          Party to the receiving Party at any time pursuant to this Agreement.

                                    ARTICLE 5

                              FINANCIAL PROVISIONS

     5.1  DISCOVERY, DEVELOPMENT AND MANUFACTURING FUNDING.

          5.1.1 Discovery and Development Funding. Biogen Idec will reimburse
               Alnylam for all internal and out-of-pocket costs and expenses
               incurred by Alnylam during the Collaboration Term in accordance
               with the Annual Work Plan, for [**]. The costs of Alnylam's
               internal personnel engaged in such efforts shall be determined by
               multiplying the number of full time equivalent person years
               ("FTEs") allocated by Alnylam to the Collaboration during the
               relevant time period, by the FTE Rate. The "FTE Rate" for work
               performed by Alnylam FTEs will be $[**] per FTE, which amount may
               be increased or decreased on an annual basis as mutually agreed
               by the Parties, but in no event more than [**] percent ([**]%)
               per Contract Year.

          5.1.2 Manufacturing Funding. Biogen Idec will pay Alnylam [**] for
               quantities of each Selected RNAi Therapeutic supplied by Alnylam
               to Biogen Idec pursuant to Section 2.10.1 that is equal to [**]
               as set forth in [**].

     5.2  MILESTONES.

          5.2.1 Upfront Payment. No later than ten (10) Business Days after the
               Effective Date, Biogen Idec will pay to Alnylam Five Million U.S.
               Dollars (U.S. $5,000,000), of


                                       22

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

               which [**] U.S. Dollars ($[**]) will be allocated to the payment
               required under Section 3.1 of the [**] Sublicense Agreement.

          5.2.2 Other Milestones. Biogen Idec will also pay to Alnylam the
               amounts set forth below no later than [**] days after the
               earliest date on which the corresponding milestone event has
               first been achieved with respect to a Selected RNAi Therapeutic.
               For clarity, in the event that a milestone event that was already
               achieved with respect to the Development Candidate is achieved
               with respect to the Back-Up Candidate, or vice versa, then no
               additional milestone payment shall be due with respect to the
               second achievement of the applicable milestone event.

<TABLE>
<CAPTION>
                                            Payment
           Milestone Event             (in U.S. Dollars)
           ---------------             -----------------
<S>                                    <C>
Initiation of IND-Enabling GLP
   Toxicology Studies                        $[**]
Upon the first IND becoming an
   Active IND                                $[**]
Clinical Proof of Concept                    $[**]
Established Utilization or
   Regulatory Approval, whichever
   occurs first, in the first Major
   Market(1)                                 $[**]
Established Utilization or
   Regulatory Approval, whichever
   occurs first, in the second Major
   Market(1)*                                $[**]
Treatment of the 100th patient               $[**]
</TABLE>

(1)  For purposes of determining achievement of these milestones only,
     Established Utilization shall be determined on a Major Market by Major
     Market basis (for example, for such purpose, the number of patients
     referred to in clause (a) of Section 1.38 shall be determined based on the
     number of such patients in a particular Major Market).

     5.3  ROYALTIES. Commencing upon the date of the first achievement of
          Established Utilization anywhere in the Territory (the "Established
          Utilization Date"), Biogen Idec will also pay to Alnylam an amount
          equal to the greater of (a) the royalties calculated in accordance
          with Section 5.3.1, and (b) the annual royalty calculated in
          accordance with Section 5.3.2:

          5.3.1 Royalties equal to the following percentage of the combined
               total of annual worldwide Net Sales of the Selected RNAi
               Therapeutics:

----------
*    If Established Utilization or Regulatory Approval in the first Major Market
     occurs in any of the United Kingdom, France, Germany, Italy or Spain, this
     milestone shall only be due if this milestone is subsequently met for the
     European Union, United States or Japan.


                                       23
<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

<TABLE>
<CAPTION>
On the portion of the combined total of annual
   worldwide Net Sales of the Selected RNAi
             Therapeutics that is:               Royalty Rate
----------------------------------------------   -------------
<S>                                              <C>
Less than $[**]                                      [**]%
From $[**] to $[**]                                  [**]%
Greater than $[**]                                   [**]%
</TABLE>

          5.3.2 An annual royalty comprising (a) a Fixed Annual Royalty for the
               Established Utilization Year in question as specified below, plus
               (b) for every patient treated with a Selected RNAi Therapeutic
               during such year, a fee of [**] U.S. Dollars ($[**]), up to an
               aggregate amount equal to the yearly Treatment Cap outlined
               below:

<TABLE>
<CAPTION>
Period                           Fixed Annual Royalty   Treatment Cap
------                           --------------------   -------------
<S>                              <C>                    <C>
Established Utilization Year 1           $[**]              $[**]
Established Utilization Year 2           $[**]              $[**]
Established Utilization Year 3           $[**]              $[**]
Established Utilization Year 4
   and beyond                            $[**]              $[**]
</TABLE>

     5.4  PAYMENTS; REPORTS.

          5.4.1 Fixed Annual Royalties/Treatment Reports and Payments. Within
               [**] days after the beginning of each Established Utilization
               Year, Biogen Idec will pay to Alnylam the Fixed Annual Royalty
               set forth in Section 5.3.2. Biogen Idec shall also, within [**]
               days after the end of each Established Utilization Quarter,
               submit to Alnylam a report stating the number of patients treated
               with each Selected RNAi Therapeutic, other than those treated in
               the context of a registrational trial, during such Established
               Utilization Quarter. Concurrently with such report, Biogen Idec
               shall pay to Alnylam the amounts due for each such patient as set
               forth in Section 5.3.2.

          5.4.2 Royalty Reports and Payments. Within [**] days after the end of
               each Established Utilization Year, Biogen Idec will provide to
               Alnylam a report, on a country-by-country basis, providing in
               reasonable detail an accounting of all Net Sales of the Selected
               RNAi Therapeutics made during such Established Utilization Year
               and the calculation of the applicable royalty under Section
               5.3.1. If such royalty report shows that the amount of royalties
               due for such Established Utilization Year exceeds the amounts
               paid by Biogen Idec to Alnylam in accordance with Section 5.4.1
               then, concurrently with such report, Biogen Idec


                                       24

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

               shall pay to Alnylam the difference between the amounts paid
               pursuant to Section 5.4.1 and the royalties due under Section
               5.3.1 for the applicable Established Utilization Year. If there
               is an Established Utilization Year where Biogen Idec is required
               to make a payment to Alnylam in accordance with the immediately
               preceding sentence then, commencing with the Established
               Utilization Year immediately following such Established
               Utilization Year, (a) Biogen Idec shall be required to provide to
               Alnylam the royalty reports required under this Section 5.4.2
               within [**] days after the end of each Established Utilization
               Quarter during each succeeding Established Utilization Year; and
               (b) if any such report shows that the cumulative royalties due
               for such Established Utilization Year exceed the cumulative
               payments made during that Established Utilization Year under
               Section 5.4.1 then, concurrently with such report, Biogen Idec
               shall pay to Alnylam the difference between the amounts paid to
               date during such Established Utilization Year pursuant to Section
               5.4.1 and the royalties due under Section 5.3.1 to date for the
               applicable Established Utilization Year.

          5.4.3 Payment Term. The payments required under Section 5.3 shall be
               effective as of the Established Utilization Date, and shall
               continue on a Selected RNAi Therapeutic-by-Selected RNAi
               Therapeutic basis until the later of (a) the expiration of the
               last Valid Claim of any Alnylam Patent Rights covering the
               Manufacture or Commercialization of a Selected RNAi Therapeutic
               in the Territory, or (b) the [**] anniversary of the Established
               Utilization Date or Regulatory Approval, whichever comes first;
               provided, however, that (i) only one royalty shall be due under
               Section 5.3.1 with respect to the same unit of Selected RNAi
               Therapeutic, and (ii) no royalties shall be due under Section
               5.3.1 upon the sale or other transfer among Biogen Idec or its
               Related Parties, but in such cases the royalty shall be due and
               calculated upon Biogen Idec's or its Related Party's Net Sales to
               the first independent Third Party.

          5.4.4 Blended Royalty Rates. The Parties acknowledge and agree that
               the Patent Rights and Know-How licensed pursuant to this
               Agreement justify royalty rates of differing amounts with respect
               to the sales of each Selected RNAi Therapeutic, which rates could
               be applied separately to each Selected RNAi Therapeutic involving
               the exercise of such Patent Rights and/or the incorporation of
               such Know-How, and that, if such royalties were calculated
               separately, royalties relating to Patent Rights and royalties
               relating to Know-How would last for different terms.
               Notwithstanding the foregoing, the Parties have determined, for
               reasons of convenience, that blended royalty rates for the Patent
               Rights and the Know-How licensed hereunder, as set forth above,
               will apply during a single payment term. The Parties acknowledge
               and agree that nothing in this Agreement (including without
               limitation any exhibits or attachments hereto) shall be construed
               as representing an estimate or projection of the anticipated
               sales or the actual value of any Selected RNAi Therapeutic, and
               that the figures set forth in this Section 5.4 or elsewhere in
               this Agreement or that have otherwise been discussed by the
               Parties are merely intended to define the Parties' royalty


                                       25

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

               payment obligations to each other in the event such sales
               performance is achieved.

          5.4.5 Royalties under [**] Sublicense Agreement. For avoidance of
               doubt, the royalties payable by Biogen Idec under this Agreement
               include any royalties due under the [**] Sublicense Agreement,
               and no additional payment will be due from Biogen Idec for such
               royalties.

     5.5  AUDITS. Each Party shall keep complete and accurate records of the
          underlying revenue, expense and other data relating to the reports and
          payments required by this Article 5. Each Party will have the right
          once annually at its own expense to have an independent, certified
          public accountant, selected by such Party and reasonably acceptable to
          the other Party, review any such records of the other Party in the
          location(s) where such records are maintained by the other Party upon
          reasonable notice and during regular business hours and under
          obligations of strict confidence, for the sole purpose of verifying
          the basis and accuracy of payments made under this Article 5, in each
          case within the prior thirty-six (36) month period. If the review of
          such records reveals that the other Party has failed to accurately
          report information pursuant to this Article 5, then the other Party
          shall promptly pay to the auditing Party any resulting amounts due
          under this Article 5, together with interest calculated in the manner
          provided in Section 5.8. If any amounts due under this Article 5 as a
          result of such audit are greater than the lesser of $[**] or [**]
          percent ([**]%) of the amounts actually due for a calendar year under
          Article 5, as applicable, the other Party shall pay all of the costs
          of such review. Each Party shall treat all financial information
          subject to review under this Section 5.5 or under any sublicense
          agreement in accordance with the confidentiality and non-use
          provisions of this Agreement, and shall cause its accounting firm to
          enter into an acceptable confidentiality agreement with the other
          Party and/or its Affiliates and Sublicensees obligating it to retain
          all such information in confidence pursuant to such confidentiality
          agreement.

     5.6  TAX MATTERS. If laws, rules or regulations require withholding of
          income taxes or other taxes imposed upon payments set forth in this
          Article 5, the paying Party shall make such withholding payments as
          required and subtract such withholding payments from the payments set
          forth in this Article 5. The paying Party shall submit appropriate
          proof of payment of the withholding taxes to the receiving Party
          within a reasonable period of time. At the request of the receiving
          Party, the paying Party shall, at the receiving party's cost, give the
          receiving Party such reasonable assistance, which shall include the
          provision of appropriate certificates of such deductions made together
          with other supporting documentation as may be required by the relevant
          tax authority, to enable the receiving Party to claim exemption from
          such withholding or other tax imposed or obtain a repayment thereof or
          reduction thereof and shall upon request provide such additional
          documentation from time to time as is reasonably required to confirm
          the payment of tax.

     5.7  PAYMENT AND CURRENCY EXCHANGE. All payments to be made under this
          Agreement shall be made in United States dollars and shall be paid by
          bank wire transfer in immediately available funds to such bank account
          in the United States as may be designated in writing by the receiving
          Party from time to time. In the case of Net Sales outside the United
          States by Biogen Idec and its Related Parties, the rate of exchange to


                                       26

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          be used in computing the amount of currency equivalent in United
          States dollars due shall be made at the rate of exchange utilized by
          Biogen Idec in its worldwide accounting system.

     5.8  LATE PAYMENTS. The paying Party shall pay interest to the receiving
          Party on the aggregate amount of any payments that are not paid on or
          before the date such payments are due under this Agreement at a rate
          per annum equal to the lesser of the prime rate of interest plus [**]
          percent ([**]%), as reported by The Wall Street Journal, or the
          highest rate permitted by applicable law, calculated on the number of
          days such payments are paid after the date such payments are due.

                                   ARTICLE 6

         INTELLECTUAL PROPERTY OWNERSHIP, PROTECTION AND RELATED MATTERS

     6.1  INVENTORSHIP. Inventorship for patentable inventions conceived or
          reduced to practice during the course of the performance of activities
          pursuant to this Agreement shall be determined in accordance with
          United States patent laws for determining inventorship.

     6.2  OWNERSHIP. Alnylam shall own the entire right, title and interest in
          and to all inventions and discoveries (and Patent Rights claiming
          patentable inventions therein) first made or discovered solely by
          employees or consultants of Alnylam or acquired solely by Alnylam in
          the course of performing its obligations under this Agreement. Biogen
          Idec shall own the entire right, title and interest in and to all
          inventions and discoveries (and Patent Rights claiming patentable
          inventions therein) first made or discovered solely by employees or
          consultants of Biogen Idec or acquired solely by Biogen Idec in the
          course of performing its obligations under this Agreement. The Parties
          shall jointly own any inventions and discoveries (and Patent Rights
          claiming patentable inventions therein) first made or discovered
          jointly in the course of performing their obligations under this
          Agreement.

     6.3  PROSECUTION AND MAINTENANCE OF PATENT RIGHTS.

          6.3.1 Biogen Idec Technology. Biogen Idec has the sole responsibility
               to, at Biogen Idec's discretion, file, conduct ex parte and inter
               partes prosecution, and maintain (including the defense of any
               interference or opposition proceedings) in the Territory, all
               Patent Rights comprising Biogen Idec Technology (other than Joint
               Collaboration IP), in Biogen Idec's name.

          6.3.2 Alnylam Technology. Alnylam has the sole responsibility to, at
               Alnylam's discretion, file, conduct ex parte and inter partes
               prosecution, and maintain (including the defense of any
               interference or opposition proceedings) in the Territory, all
               Patent Rights comprising Alnylam Technology (other than Joint
               Collaboration IP), in Alnylam's name; provided, however, that any
               filing, prosecution and maintenance of any Core JCV Patents shall
               be subject to Biogen Idec's continuing right to the prior review
               of, comment on, revision to and


                                       27

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

               approval of material documents, which shall not be unreasonably
               delayed or withheld.

          6.3.3 Joint Collaboration IP. Subject to Alnylam's continuing right to
               the prior review of, comment on, revision to and approval of
               material documents, which shall not be unreasonably delayed or
               withheld, Biogen Idec has the sole responsibility to, at Biogen
               Idec's discretion, file, conduct ex parte and inter partes
               prosecution, and maintain (including the defense of any
               interference or opposition proceedings) in the Territory, all
               Patent Rights comprising Joint Collaboration IP (other than Broad
               RNAi Technology Collaboration IP), in the names of both Alnylam
               and Biogen Idec. Alnylam shall use Commercially Reasonable
               Efforts to make available to Biogen Idec or its authorized
               attorneys, agents or representatives, such of its employees as
               Biogen Idec in its reasonable judgment deems necessary in order
               to assist it in obtaining patent protection for such Joint
               Collaboration IP. Alnylam shall sign, or use Commercially
               Reasonable Efforts to have signed, all legal documents necessary
               to file and prosecute patent applications or to obtain or
               maintain patents in respect of such Joint Collaboration IP, at no
               cost to Biogen Idec.

          6.3.4 Broad RNAi Technology Collaboration IP. Notwithstanding Section
               6.3.3, subject to Biogen Idec's continuing right to the prior
               review of, comment on, revision to and approval of material
               documents relating to Joint Collaboration IP, which shall not be
               unreasonably delayed or withheld, Alnylam has the sole
               responsibility to, at Alnylam's discretion, file, conduct ex
               parte and inter partes prosecution, and maintain, including the
               defense of any interference or opposition proceedings, in the
               Territory, all Patent Rights comprising Broad RNAi Technology
               Collaboration IP in the names of both Alnylam and Biogen Idec.
               Biogen Idec shall use Commercially Reasonable Efforts to make
               available to Alnylam or its authorized attorneys, agents or
               representatives, such of its employees as Alnylam in its
               reasonable judgment deems necessary in order to assist it in
               obtaining patent protection for such Broad RNAi Technology
               Collaboration IP. Biogen Idec shall sign or use Commercially
               Reasonable Efforts to have signed all legal documents necessary
               to file and prosecute patent applications or to obtain or
               maintain patents in respect of such Broad RNAi Technology
               Collaboration IP, at no cost to Alnylam.

          6.3.5 Contingent Rights. The Party having the right to prosecute and
               maintain patents under Sections 6.3.1, 6.3.2, 6.3.3 and 6.3.4
               shall be referred to as the "Prosecuting Party". In the event the
               Prosecuting Party elects not to seek or continue to seek or
               maintain patent protection on any Alnylam Collaboration IP,
               Biogen Idec Collaboration IP or Joint Collaboration IP which are
               subject to the other Party's licensed rights under Article 4 in
               the Territory, the other Party shall have the right (but not the
               obligation), at its expense, to prosecute and maintain in any
               country within the Territory patent protection on such Alnylam
               Collaboration IP, Biogen Idec Collaboration IP or Joint
               Collaboration IP in the name of Alnylam, Biogen Idec or both
               Parties as set forth in Sections 6.3.1, 6.3.2, 6.3.3 and 6.3.4.
               The


                                       28

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

               previously Prosecuting Party shall use Commercially Reasonable
               Efforts to make available to the other Party or its authorized
               attorneys, agents or representatives, such of its employees as
               are reasonably necessary to assist the other Party in obtaining
               and maintaining the patent protection described under this
               Section 6.3.5. The previously Prosecuting Party shall sign or use
               Commercially Reasonable Efforts to have signed all legal
               documents necessary to file and prosecute such patent
               applications or to obtain or maintain such patents.

          6.3.6 Cooperation. Each Party hereby agrees: (a) to make its
               employees, agents and consultants reasonably available to the
               other Party (or to the other Party's authorized attorneys, agents
               or representatives), to the extent reasonably necessary to enable
               such Party to undertake patent prosecution as contemplated by
               this Agreement; (b) to cooperate, if necessary and appropriate,
               with the other Party in gaining patent term extensions wherever
               applicable to Patent Rights that are subject to this; and (c) to
               endeavor in good faith to coordinate its efforts with the other
               Party to minimize or avoid interference with the prosecution and
               maintenance of the other Party's patent applications that are
               subject to this Agreement.

          6.3.7 Patent Expenses. The patent filing, prosecution and maintenance
               expenses incurred after the Effective Date with respect to Patent
               Rights comprised of Alnylam Technology and Biogen Idec Technology
               ("Patent Expenses") shall be borne by each Party having the right
               to file, prosecute and maintain such Patent Rights under this
               Section 6.3.

     6.4  THIRD PARTY INFRINGEMENT.

          6.4.1 Notices. Each Party shall promptly report in writing to the
               other Party during the Term any (a) known or suspected
               infringement of any Alnylam Technology or Biogen Idec Technology
               being used in the Collaboration, including without limitation any
               Joint Collaboration IP, or (b) unauthorized use or
               misappropriation of any Confidential Information by a Third Party
               of which it becomes aware, and shall provide the other Party with
               all available evidence supporting such infringement, or
               unauthorized use or misappropriation.

          6.4.2 Rights to Enforce.

               (a)  Biogen Idec's First Right. Subject to the provisions of
                    Section 6.4.2(b) and the provisions of any Third Party
                    agreement under which Biogen Idec's rights in Biogen Idec
                    Technology are granted or Alnylam's rights in Alnylam
                    Technology are granted and of any In-License, in respect of
                    the Selected RNAi Therapeutics in the Territory, Biogen Idec
                    shall have the sole and exclusive right to initiate an
                    infringement or other appropriate suit anywhere in the world
                    against any Third Party who at any time has infringed, or is
                    suspected of infringing, any Patent Rights, or of using
                    without proper authorization any Know-How, comprising in
                    each case (i) Biogen Idec Technology or the Core JCV
                    Patents, or (ii) Joint


                                       29

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                    Collaboration IP Covering, claiming or relating to the
                    Selected RNAi Therapeutics. Alnylam will consider in good
                    faith any request from Biogen Idec to initiate an
                    infringement or other appropriate suit anywhere in the world
                    against any Third Party who at any time has infringed, or is
                    suspected of infringing, any Patent Rights, or of using
                    without proper authorization any Know-How, comprising
                    Alnylam Technology licensed to Biogen Idec under Section 4.1
                    other than the Core JCV Patents, but Alnylam shall not be
                    required to initiate any such suit.

               (b)  Alnylam's First Right. Alnylam shall have the sole and
                    exclusive right to initiate an infringement or other
                    appropriate suit anywhere in the world against any Third
                    Party who at any time has infringed, or is suspected of
                    infringing, any Patent Rights, or of using without proper
                    authorization any Know-How, comprising (i) any Alnylam
                    Technology other than the Core JCV Patents, and (ii) Broad
                    RNAi Technology Collaboration IP.

          6.4.3 Step-In Rights. Subject to the provisions of any Third Party
               license agreement under which Alnylam's rights in Alnylam
               Technology are granted or Biogen Idec's rights in Biogen Idec
               Technology are granted, and of any In-Licenses, if the Party with
               the first right to enforce (the "Initial Enforcement Rights
               Party") Alnylam Technology, Biogen Idec Technology, Joint
               Collaboration IP or Broad RNAi Technology Collaboration IP under
               Section 6.4.2 fails to initiate a suit or take other appropriate
               action that it has the initial right to initiate or take pursuant
               thereto within [**] days after becoming aware of the basis for
               such suit or action, then the other Party (the "Secondary
               Enforcement Rights Party") may, in its discretion, provide the
               Initial Enforcement Rights Party with written notice of such
               Secondary Enforcement Rights Party's intent to initiate a suit or
               take other appropriate action. If the Secondary Enforcement
               Rights Party provides such notice and the Initial Enforcement
               Rights Party fails to initiate a suit or take such other
               appropriate action within [**] days after receipt of such notice
               from the Secondary Enforcement Rights Party, then the Secondary
               Enforcement Rights Party shall have the right to initiate a suit
               or take other appropriate action that it believes is reasonably
               required to protect its ownership interest in and to, or licensed
               interest under, as applicable, Alnylam Technology and Biogen Idec
               Technology, including without limitation, Joint Collaboration IP
               and Broad RNAi Technology Collaboration IP.

          6.4.4 Procedures; Expenses and Recoveries. The Party having the right
               to initiate any infringement suit under Section 6.4.2 or 6.4.3
               above shall have the sole and exclusive right to select counsel
               for any such suit and shall pay all expenses of the suit,
               including attorneys' fees and court costs and reimbursement of
               the other Party's reasonable out-of-pocket expense in rendering
               assistance requested by the initiating Party. If required under
               applicable law in order for the initiating Party to initiate
               and/or maintain such suit, or if either Party is unable to
               initiate or prosecute such suit solely in its own name or it is
               otherwise advisable to obtain an effective legal remedy, in each
               case, the other Party shall join as a party to the suit


                                       30

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

               and will execute and cause its Affiliates to execute all
               documents necessary for the initiating Party to initiate
               litigation to prosecute and maintain such action. In addition, at
               the initiating Party's request, the other Party shall provide
               reasonable assistance to the initiating Party in connection with
               an infringement suit at no charge to the initiating Party except
               for reimbursement by the initiating Party of reasonable
               out-of-pocket expenses incurred in rendering such assistance. The
               non-initiating Party shall have the right to participate and be
               represented in any such suit by its own counsel at its own
               expense. If the Parties obtain from a Third Party, in connection
               with such suit, any damages, license fees, royalties or other
               compensation (including any amount received in settlement of such
               litigation), such amounts shall be allocated as follows:

               (a)  In all cases, to reimburse each Party for all expenses of
                    the suit, including attorneys' fees and disbursements, court
                    costs and other litigation expenses; and

               (b)  subject to Section 6.4.4(d), if the infringement suit was
                    initiated by Biogen Idec, then any of the remaining amount
                    that relates to the JCV RNAi Product shall be treated as if
                    it were Net Sales of Biogen Idec, with Alnylam receiving a
                    royalty on such remaining amount pursuant to the terms of
                    Section 5.3.1, and the balance being retained by Biogen
                    Idec;

               (c)  subject to Section 6.4.4(d), if the infringement suit was
                    initiated by Alnylam, then Alnylam will pay to Biogen Idec
                    out of the remaining portion that relates to the JCV RNAi
                    Product an amount equal to the amount of royalties that
                    Alnylam would have received under Section 5.3.1 had such
                    portion been Net Sales of Biogen Idec, and the balance being
                    retained by Alnylam; and

               (d)  if the infringement suit was initiated by Alnylam pursuant
                    to Section 6.4.2(b), then Alnylam shall receive the full
                    amount remaining after reimbursement of the Parties pursuant
                    to Section 6.4.4(a).

     6.5  CLAIMED INFRINGEMENT.

          6.5.1 Notice. In the event that a Third Party at any time provides
               written notice of a claim to, or brings an action, suit or
               proceeding against, any Party, or any of their respective
               Affiliates or Sublicensees, claiming infringement of its patent
               rights or unauthorized use or misappropriation of its know-how,
               based upon an assertion or claim arising out of the Discovery of
               any RNAi Therapeutics directed to JCV or the Development,
               Manufacture or Commercialization of a Selected RNAi Therapeutic
               in the Territory ("Infringement Claim"), such Party shall
               promptly notify the other Party of the claim or the commencement
               of such action, suit or proceeding, enclosing a copy of the claim
               and all papers served. Each Party agrees to make available to the
               other Party its advice and counsel regarding the technical merits
               of any such claim at no cost to the other Party and to offer
               reasonable assistance to the other Party at no cost to the other
               Party.


                                       31

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          6.5.2 Right to Defend. Biogen Idec shall have the right, but not the
               obligation, to defend any Infringement Claim brought against
               either Party or its Affiliates or Sublicensees arising out of the
               Discovery, Development, Manufacture or Commercialization of a
               Selected RNAi Therapeutic in the Territory; provided, however,
               that Alnylam shall defend any such Infringement Claim relating to
               Blocking RNAi Technology or Broad RNAi Technology Controlled by
               Alnylam if it so desires. All litigation costs and expenses
               incurred by the Defending Party (defined below) in connection
               with such Infringement Claim, and all damages, payments and other
               amounts awarded against, or payable by, either Party under any
               settlement with such Third Party shall be borne by the Defending
               Party.

          6.5.3 Procedure. The Party having the obligation or first right to
               defend an Infringement Claim shall be referred to as the
               "Defending Party." The Defending Party shall have the sole and
               exclusive right to select counsel for any Infringement Claim;
               provided, that it shall consult with the other Party with respect
               to selection of counsel for such defense. The Defending Party
               shall keep the other Party informed, and shall from time to time
               consult with the other Party regarding the status of any such
               claims and shall provide the other Party with copies of all
               documents filed in, and all written communications relating to,
               any suit brought in connection with such claims. The other Party
               shall also have the right to participate and be represented in
               any such claim or related suit, at its own expense. The other
               Party shall have the sole and exclusive right to control the
               defense of an Infringement Claim in the event the Defending Party
               fails to exercise its right to assume such defense within [**]
               days following written notice from the other Party of such
               Infringement Claim. No Party shall settle any claims or suits
               involving rights of another Party without obtaining the prior
               written consent of such other Party, which consent shall not be
               unreasonably withheld.

          6.5.4 Limitations. EXCEPT AS OTHERWISE EXPRESSLY PROVIDED IN SECTION
               9.5, THE FOREGOING STATES THE ENTIRE RESPONSIBILITY OF ALNYLAM
               AND BIOGEN IDEC, AND THE SOLE AND EXCLUSIVE REMEDY OF ALNYLAM OR
               BIOGEN IDEC, AS THE CASE MAY BE, IN THE CASE OF ANY CLAIMED
               INFRINGEMENT OF ANY THIRD PARTY PATENT RIGHTS OR UNAUTHORIZED USE
               OR MISAPPROPRIATION OF ANY THIRD PARTY'S KNOW-HOW.

     6.6  OTHER INFRINGEMENT RESOLUTIONS In the event of a dispute or potential
          dispute that has not ripened into a demand, claim or suit of the types
          described in Sections 6.4 and 6.5 of this Agreement (e.g., actions
          seeking declaratory judgments and revocation proceedings), the same
          principles governing control of the resolution of the dispute, consent
          to settlements of the dispute, and implementation of the settlement of
          the dispute (including sharing in and allocating the payment or
          receipt of damages, license fees, royalties and other compensation)
          shall apply.

     6.7  PRODUCT TRADEMARKS. Biogen Idec shall own the Product Trademarks for
          each Selected RNAi Therapeutic and shall be solely responsible for
          filing and maintaining the Product Trademarks in the Territory
          (including payment of costs associated therewith). Biogen


                                       32

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          Idec shall assume full responsibility, at its sole cost and expense,
          for any infringement of a Product Trademark for a Selected RNAi
          Therapeutic by a Third Party, and shall defend and indemnify Alnylam
          for and against any claims of infringement of the rights of a Third
          Party by the use of a Product Trademark in connection with a Selected
          RNAi Therapeutic.

     6.8  PATENT TERM EXTENSIONS. The Parties shall use reasonable efforts to
          obtain all available supplementary protection certificates ("SPC") and
          other extensions of Patent Rights (including those available under the
          Hatch-Waxman Act). Each Party shall execute such authorizations and
          other documents and take such other actions as may be reasonably
          requested by the other Party to obtain such extensions. The Parties
          shall cooperate with each other in gaining patent term restorations,
          extensions and/or SPCs wherever applicable to Patent Rights. The Party
          first eligible to seek patent term restoration or extension of any
          such Patent Rights or any SPC related thereto shall have the right to
          do so; provided, that if in any country the first Party has an option
          to extend the patent term for only one of several patents, the first
          Party shall consult with the other Party before making the election.
          If more than one patent is eligible for extension or patent term
          restoration, Biogen Idec shall select in good faith a strategy that
          shall maximize patent protection and commercial value for each
          Selected RNAi Therapeutic. All filings for such extensions and
          certificates shall be made by the Party to whom responsibility for
          prosecution and maintenance of the Patent Rights are assigned,
          provided, that in the event that the Party to whom such responsibility
          is assigned elects not to file for an extension or SPC, such Party
          shall (a) inform the other Party of its intention not to file and (b)
          grant the other Party the right to file for such extension or SPC in
          the patentee's name, and (c) provide all necessary assistance in
          connection therewith.

     6.9  PATENT CERTIFICATION. To the extent required by law or permitted by
          law, the Parties shall use Commercially Reasonable Efforts to maintain
          with the applicable Regulatory Authorities during the Agreement Term
          correct and complete listings of applicable Patent Rights for the each
          Selected RNAi Therapeutic being Commercialized, including all
          so-called "Orange Book" listings required under the Hatch-Waxman Act.

                                    ARTICLE 7

                                 CONFIDENTIALITY

     7.1  CONFIDENTIAL INFORMATION. All Confidential Information disclosed by a
          Party to the other Party during the term of this Agreement shall be
          used by the receiving Party solely in connection with the activities
          contemplated by this Agreement, shall be maintained in confidence by
          the receiving Party and shall not otherwise be disclosed by the
          receiving Party to any other person, firm, or agency, governmental or
          private (other than a Party's Affiliates), without the prior written
          consent of the disclosing Party, except to the extent that the
          Confidential Information (as determined by competent documentation):

          7.1.1 was known or used by the receiving Party or its Affiliates prior
               to its date of disclosure to the receiving Party; or


                                       33

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          7.1.2 either before or after the date of the disclosure to the
               receiving Party is lawfully disclosed to the receiving Party or
               its Affiliates by sources other than the disclosing Party
               rightfully in possession of the Confidential Information; or

          7.1.3 either before or after the date of the disclosure to the
               receiving Party or its Affiliates becomes published or generally
               known to the public (including information known to the public
               through the sale of products in the ordinary course of business)
               through no fault or omission on the part of the receiving Party,
               its Affiliates or its Sublicensees; or

          7.1.4 is independently developed by or for the receiving Party or its
               Affiliates without reference to or reliance upon the Confidential
               Information.

          In addition, the provisions of this Section 7.1 shall not preclude the
          receiving Party or its Affiliates from disclosing Confidential
          Information to the extent such Confidential Information is required to
          be disclosed by the receiving Party or its Affiliates to comply with
          applicable laws, to defend or prosecute litigation or to comply with
          governmental regulations, provided that the receiving Party provides
          prior written notice of such disclosure to the disclosing Party and
          takes reasonable and lawful actions to avoid and/or minimize the
          degree of such disclosure. Specific information shall not be deemed to
          be within any of the foregoing exclusions merely because it is
          embraced by more general information falling within these exclusions.
          Alnylam and Biogen Idec each agree that they shall provide
          Confidential Information received from the other Party only to their
          respective employees, consultants and advisors, and to the employees,
          consultants and advisors of such Party's Affiliates, who have a need
          to know and have an obligation to treat such information and materials
          as confidential. All obligations of confidentiality imposed under this
          Article 7 shall expire [**] years following termination or expiration
          of this Agreement.

     7.2  PUBLICATIONS. Biogen Idec and Alnylam each acknowledge the other
          Party's interest in publishing the results of the Collaboration. Each
          Party also recognizes the mutual interest in obtaining valid patent
          protection and in protecting business interests and trade secret
          information. Consequently, either Party, its Affiliates, or their
          respective employees or consultants wishing to make a publication or a
          disclosure to a Third Party relating to an RNAi Therapeutic directed
          to JCV, a Selected RNAi Therapeutic or any other results of the
          Collaboration shall deliver to the other Party a copy of the proposed
          written publication or an outline of an oral disclosure at least [**]
          days prior to submission for publication or presentation. The
          reviewing Party shall have the right (a) to propose modifications to
          the publication or presentation for patent reasons, trade secret
          reasons or business reasons, or (b) to request a reasonable delay in
          publication or presentation in order to protect patentable
          information. If the reviewing Party requests a delay, the publishing
          Party shall delay submission or presentation for a period of [**] days
          to enable patent applications protecting each Party's rights in such
          information to be filed in accordance with Article 6 above. Upon
          expiration of such [**] day period, the publishing Party shall be free
          to proceed with the publication or presentation. If the reviewing
          Party requests modifications to the publication or presentation, the
          publishing


                                       34

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          Party shall edit such publication to prevent disclosure of trade
          secret or proprietary business information prior to submission of the
          publication or presentation.

     7.3  PUBLICITY/USE OF NAMES. No disclosure of the existence of, or the
          terms of, this Agreement may be made by either Party, and no Party
          shall use the name, trademark, trade name or logo of the other Party
          or its employees in any publicity, news release or disclosure relating
          to this Agreement or its subject matter, without the prior express
          written permission of the other Party, except as may be required by
          law or expressly permitted by the terms hereof.

          Notwithstanding the foregoing, prior to the execution of this
          Agreement by both Parties, the Parties shall agree in writing upon a
          press release publicizing the Collaboration to be issued jointly by
          the Parties upon execution of this Agreement. After such initial press
          release, neither Party shall issue a press release or public
          announcement relating to the Collaboration or this Agreement without
          the prior written approval of the other Party, which approval shall
          not be unreasonably withheld, except that a Party may (a) once a press
          release or other written statement is approved in writing by both
          Parties, make subsequent public disclosure of the information
          contained in such press release or other written statement without the
          further approval of the other Party, and (b) issue a press release or
          public announcement as required, in the reasonable judgment of such
          Party, by applicable law, including without limitation by the rules or
          regulations of the United States Securities and Exchange Commission or
          similar regulatory agency in a country other than the United States or
          of any stock exchange or Nasdaq, in each case after first notifying
          the other Party of such planned press release or public announcement
          at least [**] Business Days in advance of issuing such press release
          or making such public announcement (or, with respect to press releases
          and public announcements made pursuant to the foregoing clause (b),
          with as much advance notice as possible under the circumstances if it
          is not possible to provide notice at least [**] Business Days in
          advance) for the sole purpose of allowing the other Party to review
          the proposed press release or public announcement for the inclusion of
          Confidential Information or the use of its name.

                                    ARTICLE 8

                              TERM AND TERMINATION

     8.1  TERM; EXTENSION. This Agreement shall be effective as of the Effective
          Date and, unless terminated earlier pursuant to Section 8.2 below,
          this Agreement shall continue in effect until expiration of all
          payment obligations hereunder pursuant to Section 5.4.3 ("Agreement
          Term"). Upon expiration of the Agreement Term, all licenses of the
          Parties under Article 4 then in effect shall become fully paid-up,
          perpetual licenses.

     8.2  TERMINATION FOR CAUSE.

          8.2.1 Cause for Termination. This Agreement may be terminated at any
               time during the Term:


                                       35

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

               (a)  upon written notice by either Party if the other Party (the
                    "Breaching Party") is in breach of its material obligations
                    hereunder by causes and reasons within its control and has
                    not cured such breach within sixty (60) days after notice
                    requesting cure of the breach; or

               (b)  by either Party upon the filing or institution of
                    bankruptcy, reorganization, liquidation or receivership
                    proceedings, or upon an assignment of a substantial portion
                    of the assets for the benefit of creditors by the other
                    Party; provided, however, that in the event of any
                    involuntary bankruptcy or receivership proceeding such right
                    to terminate shall only become effective if the Party
                    consents to the involuntary bankruptcy or receivership or
                    such proceeding is not dismissed within ninety (90) days
                    after the filing thereof.

          8.2.2 Effect of Termination for Cause.

               (a)  Termination by Alnylam. Without limiting any other legal or
                    equitable remedies that Alnylam may have, if Biogen Idec is
                    the Breaching Party and Alnylam terminates this Agreement in
                    accordance with Section 8.2.1(a), then

                    (i)  the license granted to Alnylam pursuant to Section
                         4.1.5 shall survive and the licenses granted to Biogen
                         Idec under this Agreement shall terminate;

                    (ii) in the event that Alnylam is Manufacturing and
                         supplying a Selected RNAi Therapeutic pursuant to
                         Section 2.10.1, Biogen Idec shall have the obligation
                         to reimburse Alnylam for any committed and
                         non-refundable or non-creditable costs or expenses
                         incurred by the Alnylam, as of the date of notice of
                         termination, with respect to the supply of each such
                         Selected RNAi Therapeutic, and shall purchase, at the
                         transfer price pursuant to Section 5.1.2, any Selected
                         RNAi Therapeutic Manufactured and supplied by Alnylam,
                         as well as any work in progress, raw materials,
                         intermediates or components relating to such Selected
                         RNAi Therapeutic; and

               (b)  Termination by Biogen Idec. Without limiting any other legal
                    or equitable remedies that Biogen Idec may have, if Alnylam
                    is the Breaching Party and Biogen Idec terminates this
                    Agreement in accordance with Section 8.2.1(a), then

                    (i)  the licenses granted to Alnylam under this Agreement
                         shall terminate and the licenses granted to Biogen Idec
                         under this Agreement shall continue in full force and
                         effect; provided, however, that to the extent such
                         licenses include a sublicense of


                                       36

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                         Know-How or Patent Rights owned or controlled by a
                         Third Party, including without limitation the Existing
                         Alnylam In-Licenses, Biogen Idec shall be fully
                         responsible for all royalties, milestones or other
                         payments under such In-Licenses reasonably allocable to
                         the Selected RNAi Therapeutics; and

                    (ii) in the event that Alnylam is Manufacturing and
                         supplying a Selected RNAi Therapeutic pursuant to
                         Section 2.10.1, Alnylam shall have the obligation, if
                         requested by Biogen Idec, to continue to Manufacture
                         and supply such Selected RNAi Therapeutic in accordance
                         with the terms of the applicable supply agreement.
                         Alnylam will cooperate in any reasonable manner
                         requested by Biogen Idec to achieve a smooth transition
                         of the Development and Manufacturing of each such
                         Selected RNAi Therapeutic to Biogen Idec or Biogen
                         Idec's licensees.

               (c)  Termination upon Bankruptcy of a Party. If this Agreement is
                    terminated by either Party (the "Non-Bankrupt Party")
                    pursuant to Section 8.2.1(b) due to the rejection of this
                    Agreement by or on behalf of the other Party (the "Bankrupt
                    Party") under Section 365 of the United States Bankruptcy
                    Code (the "Code"), all licenses and rights to licenses
                    granted under or pursuant to this Agreement by the Bankrupt
                    Party to the Non-Bankrupt Party are, and shall otherwise be
                    deemed to be, for purposes of Section 365(n) of the Code,
                    licenses of rights to "intellectual property" as defined
                    under Section 101(35A) of the Code. The Parties agree that
                    the Non-Bankrupt Party, as a licensee of such rights under
                    this Agreement, shall retain and may fully exercise all of
                    its rights and elections under the Code, and that upon
                    commencement of a bankruptcy proceeding by or against the
                    Bankrupt Party under the Code, the Non-Bankrupt Party shall
                    be entitled to a complete duplicate of, or complete access
                    to (as the Non-Bankrupt Party deems appropriate), any such
                    intellectual property and all embodiments of such
                    intellectual property. Such intellectual property and all
                    embodiments thereof shall be promptly delivered to the
                    Non-Bankrupt Party (i) upon any such commencement of a
                    bankruptcy proceeding upon written request therefor by the
                    Non-Bankrupt Party, unless the Bankrupt Party elects to
                    continue to perform all of its obligations under this
                    Agreement or (ii) if not delivered under (i) above, upon the
                    rejection of this Agreement by or on behalf of the Bankrupt
                    Party upon written request therefor by the Non-Bankrupt
                    Party. The foregoing provisions are without prejudice to any
                    rights the Non-Bankrupt Party may have arising under the
                    Code or other applicable law.

     8.3  TERMINATION FOR CONVENIENCE. Biogen Idec shall have the right to
          terminate this Agreement, on a country-by-country basis or in its
          entirety, for any reason upon ninety (90) days' advance written notice
          to Alnylam after completion of the Discovery Term. During the
          Discovery Term, Biogen Idec shall have the right to terminate this


                                       37

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          Agreement, on a country-by-country basis or in its entirety, for any
          reason upon four (4) months' advance written notice to Alnylam.

     8.4  EFFECT OF EXPIRATION OR TERMINATION; SURVIVAL. Expiration or
          termination of this Agreement shall not relieve the Parties of any
          obligation accruing prior to such expiration or termination. Any
          expiration or termination of this Agreement shall be without prejudice
          to the rights of either Party against the other accrued or accruing
          under this Agreement prior to expiration or termination, including
          without limitation the obligation to make payments in respect of
          Selected RNAi Therapeutics sold or used prior to such expiration or
          termination. The provisions of Articles 1, 6, 7, 10, and Sections
          4.2.6, 8.2.2, 8.4 and 9.5 shall survive any expiration or termination
          of this Agreement. In addition, Section 4.1.5 will survive expiration
          or termination of this Agreement for any reason other than Alnylam's
          breach. Except as set forth in this Article 8, upon termination or
          expiration of this Agreement all other rights and obligations cease.

                                    ARTICLE 9

                 REPRESENTATIONS AND WARRANTIES; INDEMNIFICATION

     9.1  MUTUAL REPRESENTATIONS AND WARRANTIES. Each Party represents and
          warrants to the other Party that as of the Effective Date of this
          Agreement:

          9.1.1 It is duly organized and validly existing under the laws of its
               jurisdiction of incorporation or formation, and has full
               corporate or other power and authority to enter into this
               Agreement and to carry out the provisions hereof. Further, except
               for any Regulatory Approvals, pricing and/or reimbursement
               approvals, manufacturing approvals and/or similar approvals
               necessary for the Development, Manufacture or Commercialization
               of the Selected RNAi Therapeutics, all necessary consents,
               approvals and authorizations of all government authorities
               required to be obtained by such Party as of the Effective Date in
               connection with the execution, delivery and performance of this
               Agreement have been obtained by the Effective Date.

          9.1.2 It is duly authorized to execute and deliver this Agreement and
               to perform its obligations hereunder, and the person or persons
               executing this Agreement on its behalf has been duly authorized
               to do so by all requisite corporate action.

          9.1.3 This Agreement is legally binding upon it and enforceable in
               accordance with its terms. Except as set forth in Section 9(a) of
               Schedule 9 to this Agreement, the execution, delivery and
               performance of this Agreement by it does not conflict with any
               agreement, instrument or understanding, oral or written, to which
               it is a party and by which it may be bound.

          9.1.4 Except, in Alnylam's case, as set forth in Section 9(a) of
               Schedule 9 to this Agreement, it has not, and will not during the
               Agreement Term, grant any right to any Third Party which would
               conflict with the rights granted to the other Party


                                       38

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

               hereunder. It has (or will have at the time performance is due)
               maintained and will maintain and keep in full force and effect
               all agreements (including license agreements) and filings
               (including patent filings) necessary in such Party's reasonable
               judgment to perform its obligations hereunder. Further, the
               execution and delivery of this Agreement by such Party, the
               performance of such Party's obligations hereunder and the
               licenses and sublicenses to be granted by such Party pursuant to
               this Agreement do not conflict with or violate any requirement of
               applicable laws or regulations existing as of the Effective Date
               and applicable to such Party.

          9.1.5 Neither Party nor any of its Affiliates has been debarred or is
               subject to debarment and neither Party nor any of its Affiliates
               will use in any capacity, in connection with the Discovery,
               Development, Manufacture or Commercialization of an RNAi
               Therapeutic under this Agreement, any person or entity that has
               been debarred pursuant to Section 306 of the United States
               Federal Food, Drug, and Cosmetic Act, or that is the subject of a
               conviction described in such section. Each Party agrees to inform
               the other Party in writing immediately if it or any person or
               entity that is performing activities under the Collaboration is
               debarred or is the subject of a conviction described in Section
               306, or if any action, suit, claim, investigation or legal or
               administrative proceeding is pending or, to the best of such
               Party's knowledge, is threatened, relating to the debarment or
               conviction of such Party or any person or entity used in any
               capacity by such Party or any of its Affiliates in connection
               with the Discovery, Development, Manufacture or Commercialization
               of an RNAi Therapeutic under this Agreement.

     9.2  ALNYLAM REPRESENTATIONS AND WARRANTIES. Alnylam represents and
          warrants to Biogen Idec that as of the Effective Date of this
          Agreement:

          9.2.1 Alnylam Controls the Alnylam Patent Rights and is entitled to
               grant the licenses specified herein. Subject to Section 4.5, the
               Alnylam Patent Rights constitute all of the Patent Rights
               Controlled by Alnylam as of the Effective Date that relate to or
               are necessary to practice the Alnylam Patent Rights. Except as
               set forth in Section 9(a) of Schedule 9 to this Agreement,
               Alnylam has not previously assigned, transferred, conveyed or
               otherwise encumbered its right, title and interest in the Alnylam
               Technology in a manner that conflicts with any rights granted to
               Biogen Idec hereunder. During the Term of this Agreement, Alnylam
               shall not encumber the rights granted to Biogen Idec hereunder
               with respect to the Alnylam Patent Rights.

          9.2.2 To the best knowledge of Alnylam and its Affiliates, except as
               set forth in Schedule 9(c), there is no actual or threatened
               infringement of the Alnylam Patent Rights in the Field by any
               Third Party or any other infringement or threatened infringement
               that would adversely affect Biogen Idec's rights under this
               Agreement.

          9.2.3 To the best knowledge of Alnylam and its Affiliates, the Alnylam
               Patent Rights are existing as of the Effective Date are
               subsisting and are not invalid or


                                       39

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

               unenforceable, in whole or in part. The conception, development
               and reduction to practice of the Alnylam Patent Rights existing
               as of the Effective Date have not constituted or involved the
               misappropriation of trade secrets or other rights or property of
               any Person. There are no claims, judgments or settlements against
               or amounts with respect thereto owed by Alnylam or any of its
               Affiliates relating to the Alnylam Patent Rights. Except as set
               forth in Section 9(b) of Schedule 9 to this Agreement, no claim
               or litigation has been brought or threatened by any Third Party
               alleging, and Alnylam is not aware of any reasonable basis for a
               claim, whether or not asserted, that (a) the Alnylam Patent
               Rights are invalid or unenforceable or (b) the Alnylam Patent
               Rights or the disclosing, copying, making, assigning, licensing
               or exploiting of the Alnylam Patent Rights violates, infringes or
               otherwise conflicts or interferes with any intellectual property
               or proprietary right of any Third Party.

          9.2.4 To the best of Alnylam's actual knowledge, except as set forth
               in Section 9(b) of Schedule 9 to this Agreement, there are no
               claims, judgments or settlements against or owed by Alnylam or
               its Affiliates or pending or threatened claims or litigation
               relating to the Alnylam Technology that are expected to impact
               the Collaboration or any RNAi Therapeutic directed to JCV.

     9.3  BIOGEN IDEC REPRESENTATIONS AND WARRANTIES. Biogen Idec represents and
          warrants to Alnylam that as of the Effective Date of this Agreement:

          9.3.1 To the best of Biogen Idec's knowledge, the Biogen Idec Patent
               Rights exist and are not invalid or unenforceable, in whole or in
               part;

          9.3.2 It has not previously assigned, transferred, conveyed or
               otherwise encumbered its right, title and interest in the Biogen
               Idec Technology in a manner that conflicts with the rights
               granted to Alnylam hereunder; and

          9.3.3 There are no claims, judgments or settlements against or owed by
               Biogen Idec or its Affiliates or pending or threatened claims or
               litigation relating to the Biogen Idec Technology that are
               expected to impact the Collaboration or any RNAi Therapeutic
               directed to JCV.

     9.4  WARRANTY DISCLAIMER. EXCEPT AS OTHERWISE EXPRESSLY PROVIDED IN THIS
          AGREEMENT, NEITHER PARTY MAKES ANY REPRESENTATION OR EXTENDS ANY
          WARRANTY OF ANY KIND, EITHER EXPRESS OR IMPLIED, TO THE OTHER PARTY
          WITH RESPECT TO ANY TECHNOLOGY, RNAi THERAPEUTICS, GOODS, SERVICES,
          RIGHTS OR OTHER SUBJECT MATTER OF THIS AGREEMENT AND HEREBY DISCLAIMS
          ALL IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
          PURPOSE AND NONINFRINGEMENT WITH RESPECT TO ANY AND ALL OF THE
          FOREGOING. EACH PARTY HEREBY DISCLAIMS ANY REPRESENTATION OR WARRANTY
          THAT THE DEVELOPMENT, MANUFACTURE OR COMMERCIALIZATION OF ANY RNAi
          THERAPEUTIC UNDER THIS AGREEMENT WILL BE SUCCESSFUL.


                                       40

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

     9.5  INDEMNIFICATION AND INSURANCE.

          9.5.1 Indemnification by Biogen Idec. Biogen Idec shall indemnify,
               hold harmless, and defend Alnylam, its Affiliates, and their
               respective directors, officers, employees and agents ("Alnylam
               Indemnitees") from and against any and all Third Party claims,
               suits, losses, liabilities, damages, costs, fees and expenses
               (including reasonable attorneys' fees) (collectively, "Losses")
               arising out of or resulting from, directly or indirectly, (a) any
               breach of, or inaccuracy in, any representation or warranty made
               by Biogen Idec in this Agreement, or any breach or violation of
               any covenant or agreement of Biogen Idec in or pursuant to this
               Agreement, (b) the negligence or willful misconduct by or of
               Biogen Idec, its Affiliates and their respective Sublicensees,
               and their respective directors, officers, employees and agents,
               or (c) the Discovery, Development, Manufacture or
               Commercialization of a JCV RNAi Product to the extent such
               activities are not performed by Alnylam or its agents. This
               indemnification excludes Losses arising out of Third Party
               Infringement Claims resulting from Biogen Idec's exercise in
               accordance with the terms of this Agreement of any intellectual
               property rights granted by Alnylam hereunder. Furthermore, Biogen
               Idec shall have no obligation to indemnify the Alnylam
               Indemnitees to the extent that the Losses arise out of or result
               from, directly or indirectly, any breach of, or inaccuracy in,
               any representation or warranty made by Alnylam in this Agreement,
               or any breach or violation of any covenant or agreement of
               Alnylam in or pursuant to this Agreement, or the negligence or
               willful misconduct by or of any of the Alnylam Indemnitees,
               and/or the Discovery, Development or Manufacture of a JCV RNAi
               Therapeutic, to the extent performed such activities are
               performed by Alnylam or its agents.

          9.5.2 Indemnification by Alnylam. Alnylam shall indemnify, hold
               harmless, and defend Biogen Idec, its Affiliates and their
               respective directors, officers, employees and agents ("Biogen
               Idec Indemnitees") from and against any and all Losses arising
               out of or resulting from, directly or indirectly, (a) any breach
               of, or inaccuracy in, any representation or warranty made by
               Alnylam in this Agreement, or any breach or violation of any
               covenant or agreement of Alnylam in or pursuant to this
               Agreement, (b) the negligence or willful misconduct by or of
               Alnylam, its Affiliates and their respective Sublicensees, and
               their respective directors, officers, employees and agents or (c)
               the Discovery, Development or Manufacture of a JCV RNAi
               Therapeutic, to the extent performed by Alnylam or its agents.
               This indemnification excludes Losses arising out of Third Party
               Infringement Claims resulting from Alnylam's exercise in
               accordance with the terms of this Agreement of any intellectual
               property rights granted by Biogen Idec hereunder. Furthermore,
               Alnylam shall have no obligation to indemnify the Biogen Idec
               Indemnitees to the extent that the Losses arise out of or result
               from, directly or indirectly, any breach of, or inaccuracy in,
               any representation or warranty made by Biogen Idec in this
               Agreement, or any breach or violation of any covenant or
               agreement of Biogen Idec in or pursuant to this Agreement, or the
               negligence or willful misconduct by or of any of the Biogen Idec
               Indemnitees, and/or the Discovery, Development, Manufacture or
               Commercialization of a JCV


                                       41

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

               RNAi Product to the extent such activities are not performed by
               Alnylam or its agents.

          9.5.3 Indemnification Procedure. In the event of any such claim
               against any Biogen Idec Indemnitee or Alnylam Indemnitee
               (individually, an "Indemnitee"), the indemnified Party shall
               promptly notify the other Party in writing of the claim and the
               indemnifying Party shall manage and control, at its sole expense,
               the defense of the claim and its settlement. The Indemnitee shall
               cooperate with the indemnifying Party and may, at its option and
               expense, be represented in any such action or proceeding. The
               indemnifying Party shall not be liable for any settlements,
               litigation costs or expenses incurred by any Indemnitee without
               the indemnifying Party's written authorization. Notwithstanding
               the foregoing, if the indemnifying Party believes that any of the
               exceptions to its obligation of indemnification of the
               Indemnitees set forth in Sections 9.5.1 or 9.5.2 may apply, the
               indemnifying Party shall promptly notify the Indemnitees, which
               shall then have the right to be represented in any such action or
               proceeding by separate counsel at their expense; provided, that
               the indemnifying Party shall be responsible for payment of such
               expenses if the Indemnitees are ultimately determined to be
               entitled to indemnification from the indemnifying Party.

          9.5.4 Insurance. Each Party shall use all Commercially Reasonable
               Efforts to maintain insurance, including product liability
               insurance, with respect to its activities hereunder. Such
               insurance shall be in such amounts and subject to such
               deductibles as the Parties may agree based upon standards
               prevailing in the industry at the time. At such time as the any
               Selected RNAi Therapeutic is being manufactured by Biogen Idec
               for commercial sale, Biogen Idec shall name Alnylam as an
               additional insured on any such policies.

                                   ARTICLE 10

                            MISCELLANEOUS PROVISIONS

     10.1 GOVERNING LAW. This Agreement shall be construed and the respective
          rights of the Parties determined according to the substantive laws of
          the Commonwealth of Massachusetts notwithstanding the provisions
          governing conflict of laws under such Massachusetts law to the
          contrary.

     10.2 ASSIGNMENT.

          10.2.1 In General. Except as provided in this Section 10.2, this
               Agreement may not be assigned or otherwise transferred, nor may
               any right or obligation hereunder be assigned or transferred, by
               either Party without the consent of the other Party. However,
               either Party may, without the other Party's consent, assign this
               Agreement and its rights and obligations hereunder in whole or in
               part to an Affiliate or to a party that acquires, by merger, sale
               of assets or otherwise, all or substantially all of the business
               of the assigning Party to which the subject matter of this
               Agreement relates. The assigning Party shall remain responsible
               for the


                                       42

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

               performance by its assignee of this Agreement or any obligations
               hereunder so assigned to such assignee.

          10.2.2 [**].

     10.3 AMENDMENTS. This Agreement, the Schedules referred to in this
          Agreement and the [**] Sublicense Agreement constitute the entire
          agreement between the Parties with respect to the subject matter
          hereof, and supersede all previous arrangements with respect to the
          subject matter hereof, whether written or oral. Any amendment or
          modification to this Agreement shall be made in writing signed by both
          Parties.

     10.4 NOTICES. Any notice required or provided for by the terms of this
          Agreement shall be in writing and shall be (a) sent by registered or
          certified mail, return receipt requested, postage prepaid, (b) sent
          via a reputable overnight courier service, or (c) sent by facsimile
          transmission, in each case properly addressed in accordance with the
          paragraphs below. The effective date of notice shall be the actual
          date of receipt by the Party receiving the same.

               Notices to Alnylam shall be addressed to:

                    Alnylam Pharmaceuticals, Inc.
                    300 Third Street, Third Floor
                    Cambridge, Massachusetts 02141
                    Attention: Chief Executive Officer
                    Facsimile No.: (617) 551-8101

               with a copy to:

                    FABER DAEUFER & ROSENBERG, P.C.
                    950 Winter Street, Suite 4500
                    Waltham, MA 02451
                    Attention: Sumy C. Daeufer
                    Facsimile No.: (781) 795-4747

               Notices to Biogen Idec shall be addressed to:

                    Biogen Idec MA, Inc.
                    14 Cambridge Center
                    Cambridge, MA 02142
                    Attention: _________________________
                    Facsimile No.: (617) _______________

               with a copy to:

                    Ropes & Gray
                    One International Place
                    Boston, MA 02110
                    Attention: Marc A. Rubenstein


                                       43

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                    Facsimile No.: (617) 951-7050

               Either Party may change its address to which notices shall be
               sent by giving notice to the other Party in the manner herein
               provided.

     10.5 FORCE MAJEURE. No failure or omission by either Party in the
          performance of any obligation of this Agreement shall be deemed a
          breach of this Agreement or create any liability if the same shall
          arise from any cause or causes beyond the control of such Party,
          including, but not limited to, the following: acts of god; acts or
          omissions of any government; any rules, regulations or orders issued
          by any governmental authority or by any officer, department, agency or
          instrumentality thereof; fire; storm; flood; earthquake; accident;
          war; rebellion; insurrection; riot; and invasion; provided that such
          failure or omission resulting from one of the above causes is cured as
          soon as is practicable.

     10.6 COMPLIANCE WITH EXPORT REGULATIONS. Neither Party shall export any
          technology licensed to it by the other Party under this Agreement
          except in compliance with U.S. export laws and regulations.

     10.7 INDEPENDENT CONTRACTORS. It is understood and agreed that the
          relationship between the Parties is that of independent contractors
          and that nothing in this Agreement shall be construed as authorization
          for either Alnylam or Biogen Idec to act as agent for the other.
          Nothing herein contained shall be deemed to create an employment,
          agency, joint venture or partnership relationship between the Parties
          or any of their agents or employees for any purpose, including tax
          purposes, or to create any other legal arrangement that would impose
          liability upon one Party for the act or failure to act of the other
          Party. Neither Party shall have any express or implied power to enter
          into any contracts or commitments or to incur any liabilities in the
          name of, or on behalf of, the other Party, or to bind the other Party
          in any respect whatsoever.

     10.8 NO STRICT CONSTRUCTION. This Agreement has been prepared jointly and
          shall not be strictly construed against either Party.

     10.9 HEADINGS. The captions or headings of the sections or other
          subdivisions hereof are inserted only as a matter of convenience or
          for reference and shall have no effect on the meaning of the
          provisions hereof.

     10.10 NO IMPLIED WAIVERS; RIGHTS CUMULATIVE. No failure on the part of
          Alnylam or Biogen Idec to exercise, and no delay in exercising, any
          right, power, remedy or privilege under this Agreement, or provided by
          statute or at law or in equity or otherwise, shall impair, prejudice
          or constitute a waiver of any such right, power, remedy or privilege
          or be construed as a waiver of any breach of this Agreement or as an
          acquiescence therein, nor shall any single or partial exercise of any
          such right, power, remedy or privilege preclude any other or further
          exercise thereof or the exercise of any other right, power, remedy or
          privilege.

     10.11 SEVERABILITY. If any provision hereof should be held invalid, illegal
          or unenforceable in any respect in any jurisdiction, the Parties
          hereto shall substitute, by mutual consent,


                                       44

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          valid provisions for such invalid, illegal or unenforceable
          provisions, which valid provisions in their economic effect are
          sufficiently similar to the invalid, illegal or unenforceable
          provisions that it can be reasonably assumed that the Parties would
          have entered into this Agreement with such valid provisions. In case
          such valid provisions cannot be agreed upon, the invalid, illegal or
          unenforceable of one or several provisions of this Agreement shall not
          affect the validity of this Agreement as a whole, unless the invalid,
          illegal or unenforceable provisions are of such essential importance
          to this Agreement that it is to be reasonably assumed that the Parties
          would not have entered into this Agreement without the invalid,
          illegal or unenforceable provisions.

     10.12 EXECUTION IN COUNTERPARTS. This Agreement may be executed in
          counterparts, each of which counterparts, when so executed and
          delivered, shall be deemed to be an original, and all of which
          counterparts, taken together, shall constitute one and the same
          instrument.

     10.13 NO THIRD PARTY BENEFICIARIES. Except as set forth in Section 10.2.2.,
          no person or entity other than Biogen Idec, Alnylam and their
          respective Affiliates and permitted assignees hereunder shall be
          deemed an intended beneficiary hereunder or have any right to enforce
          any obligation of this Agreement.

     10.14 NO CONSEQUENTIAL DAMAGES. NEITHER PARTY HERETO WILL BE LIABLE FOR
          SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THIS
          AGREEMENT OR THE EXERCISE OF ITS RIGHTS HEREUNDER, INCLUDING WITHOUT
          LIMITATION LOST PROFITS ARISING FROM OR RELATING TO ANY BREACH OF THIS
          AGREEMENT, REGARDLESS OF ANY NOTICE OF SUCH DAMAGES. NOTHING IN THIS
          SECTION 10.14 IS INTENDED TO LIMIT OR RESTRICT THE INDEMNIFICATION
          RIGHTS OR OBLIGATIONS OF EITHER PARTY OR TO LIMIT A PARTY'S LIABILITY
          FOR BREACHES OF ITS OBLIGATION REGARDING CONFIDENTIALITY UNDER ARTICLE
          7.

          IN WITNESS WHEREOF, the Parties have executed this Agreement as of the
date first set forth above.

                                        ALNYLAM PHARMACEUTICALS, INC.


                                        By: /s/ John M. Maraganore
                                            ------------------------------------
                                        Title: President and Chief
                                               Executive Officer


                                        BIOGEN IDEC MA, INC.


                                        By: /s/ James C. Mullen
                                            ------------------------------------
                                        Title: President and Chief
                                               Executive Officer


                                       45

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                                  SCHEDULE 1.6

                              ALNYLAM PATENT RIGHTS

          1.6.1 PATENTS AND PATENT APPLICATIONS OWNED BY ALNYLAM OR LICENSED
               FROM THIRD PARTIES OTHER THAN ISIS PHARMACEUTICALS, INC.

          Alnylam Patent Rights include all claims of the patents and patent
          applications listed in Schedule 1.6.1, attached, explicitly excluding
          [**]. The patents and patent applications listed in Schedule 1.6.1 are
          either owned by Alnylam or licensed from Third Parties other than Isis
          Pharmaceuticals, Inc. ("Isis").

          1.6.2 PATENTS AND PATENT APPLICATIONS LICENSED FROM ISIS

          Alnylam Patent Rights also include rights to a broad portfolio of
          patents and patent applications (the "Isis Patents") licensed to
          Alnylam by Isis pursuant to the Strategic Collaboration & License
          Agreement between Isis and Alnylam dated March 11, 2004 (the "Isis
          Agreement"). The Isis Patents are described in Schedules 1-27 and 1-28
          of the Isis Agreement and are attached to this Agreement as Schedule
          1.6.2. Schedule 1.6.2(a) lists those Isis Patents classified in the
          Isis Agreement as "Isis Current Motif and Mechanism Patents" and
          listed in Schedule 1-28 of the Isis Agreement. Schedule 1.6.2(b) lists
          those Isis Patents classified in the Isis Agreement as "Isis Current
          Chemistry Patents" and listed in Schedule 1-27 of the Isis Agreement.


                                       46
<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                                 SCHEDULE 1.6.1

PATENTS AND PATENT APPLICATIONS OWNED BY ALNYLAM OR LICENSED FROM THIRD PARTIES
                      OTHER THAN ISIS PHARMACEUTICALS, INC.

          Confidential Materials omitted and filed separately with the
         Securities and Exchange Commission. Asterisks denote omissions

<TABLE>
<CAPTION>
                                    Application                 Publication   Publication                Issue
Case No.   Title   Country   Type       No.       Filing Date       No.           Date      Patent No.    Date
--------   -----   -------   ----   -----------   -----------   -----------   -----------   ----------   -----
<S>        <C>     <C>       <C>    <C>           <C>           <C>           <C>           <C>          <C>
[**]        [**]     [**]    [**]       [**]          [**]          [**]          [**]

                     [**]    [**]       [**]          [**]          [**]          [**]

                     [**]    [**]       [**]          [**]

                     [**]    [**]       [**]          [**]
</TABLE>


                                       47

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                                 SCHEDULE 1.6.2

     PATENT AND PATENT APPLICATIONS LICENSED FROM ISIS PHARMACEUTICALS, INC.

           SCHEDULE 1.6.2(A): ISIS CURRENT MOTIF AND MECHANISM PATENTS

          Confidential Materials omitted and filed separately with the
         Securities and Exchange Commission. Asterisks denote omissions

<TABLE>
<CAPTION>
ISIS DOCKET                      PATENT   GRANT            3RD     3RD
   NUMBER     COUNTRY   STATUS   NUMBER    DATE   TITLE   PARTY   PARTY
-----------   -------   ------   ------   -----   -----   -----   -----
<S>           <C>       <C>      <C>      <C>     <C>     <C>     <C>
[**]            [**]     [**]              [**]

[**]            [**]     [**]              [**]

[**]            [**]     [**]              [**]

[**]            [**]     [**]              [**]

[**]            [**]     [**]              [**]

[**]            [**]     [**]              [**]

[**]            [**]     [**]              [**]

[**]            [**]     [**]              [**]

[**]            [**]     [**]              [**]

[**]            [**]     [**]              [**]
</TABLE>


                                       48

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                    SCHEDULE 1.6.2(B): ISIS CHEMISTRY PATENTS

          Confidential Materials omitted and filed separately with the
         Securities and Exchange Commission. Asterisks denote omissions

<TABLE>
<CAPTION>
ISIS DOCKET   COUNTRY            PATENT   GRANT            3RD     3RD
   NUMBER       NAME    STATUS   NUMBER    DATE   TITLE   PARTY   PARTY
-----------   -------   ------   ------   -----   -----   -----   -----
<S>           <C>       <C>      <C>      <C>     <C>     <C>     <C>
[**]            [**]     [**]     [**]     [**]    [**]    [**]

[**]            [**]     [**]     [**]     [**]    [**]    [**]

[**]            [**]     [**]     [**]     [**]    [**]    [**]

[**]            [**]     [**]     [**]     [**]    [**]    [**]

[**]            [**]     [**]     [**]     [**]    [**]    [**]

[**]            [**]     [**]     [**]     [**]    [**]    [**]

[**]            [**]     [**]     [**]     [**]    [**]    [**]

[**]            [**]     [**]     [**]     [**]    [**]    [**]

[**]            [**]     [**]     [**]     [**]    [**]    [**]

[**]            [**]     [**]                      [**]    [**]

[**]            [**]     [**]     [**]     [**]    [**]    [**]

[**]            [**]     [**]     [**]     [**]    [**]    [**]    [**]

[**]            [**]     [**]     [**]     [**]    [**]    [**]

[**]            [**]     [**]     [**]     [**]    [**]    [**]
</TABLE>


                                       49

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                                  SCHEDULE 1.8
                                ANNUAL WORK PLAN

OVERALL OBJECTIVE

Develop an RNAi therapeutic that inhibits JCV for the treatment of PML.

MOLECULAR HYPOTHESIS

[**]

THERAPEUTIC HYPOTHESIS

[**]

TECHNICAL RATIONALE

[**]

OVERVIEW OF ANNUAL WORK PLAN - DISCOVERY PROGRAM, YEAR 1

This Annual Work Plan describes the Discovery Program, or candidate
identification phase, from now (Q4 2006) until [**]. The Development Program, or
pre-clinical development phase, will comprise [**] studies that commence in [**]
after [**], such that the IND can be filed in [**]. Thus, CMC activities will
begin [**] prior to [**], and are [**].

DETAILS OF ANNUAL WORK PLAN - DISCOVERY PROGRAM, YEAR 1

The primary objective of the Discovery Program (Year 1) is to [**] A second
major objective of Year 1 comprises [**]. Thus, the [**] goals for Year 1 are:

[**] Goals for Year 1 [**]

[**] Goals for Year 1

[**]

Development Criteria

The target profile for RNAi Therapeutics directed to JCV is:

[**]

The major goals for Year 1 are described in further detail below.

[**]

We will begin by [**]

[**]

We will [**]. In order to [**]. We will use[**]. We will assess [**] After a
[**]


                                       50

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

Based upon the [**]

The process consists of [**].

The components for the [**]

The [**] processes will be [**].

Timeline and Budget

The table below provides the major goals and tasks for the Discovery Program,
Year 1 (Q1-Q4) as well as the Development Program, along with projected
associated timelines, and projected associated internal resources (FTEs) and
external expenses. [**]

The projected budget for Year 1 follows. Note that there may be additional
costs, that would need further JSC approval, associated with [**]


                                       51

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                                  SCHEDULE 1.11
                             BIOGEN IDEC IN-LICENSES


                                       52

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                                  SCHEDULE 1.27
                                CORE JCV PATENTS

                                      [**]


                                       53

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                                  SCHEDULE 1.41

                          EXISTING ALNYLAM IN-LICENSES

Existing Alnylam In-Licenses shall include the following Third Party agreements:

1.   [**].

2.   [**].


                                       54

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                                  SCHEDULE 4.5
                            [**] SUBLICENSE AGREEMENT

                                LICENSE AGREEMENT

This LICENSE AGREEMENT (this "License Agreement"), effective as of September 20,
2006 (the "Effective Date"), is by and between Biogen Idec MA Inc., a
corporation organized and existing under the laws of the Commonwealth of
Massachusetts and having its principal office at 14 Cambridge Center, Cambridge,
MA 02142 ("Biogen Idec"), and Alnylam Pharmaceuticals, Inc., a corporation
organized and existing under the laws of Delaware, with its principal place of
business at 300 Third Street, 3rd Floor, Cambridge, Massachusetts 02142
("Alnylam"). Biogen Idec and Alnylam are each sometimes referred to herein
individually as a "Party" and collectively as "Parties."

                                    RECITALS

          WHEREAS, Biogen Idec and Alnylam have entered into the Collaboration
and License Agreement (the "Collaboration Agreement") as of the Effective Date;

          WHEREAS, Alnylam U.S., Inc. (then known as Alnylam Pharmaceuticals,
Inc.) is a party to that certain License Agreement [**].

          WHEREAS, Alnylam wishes to sublicense to Biogen Idec, and Biogen Idec
wishes to sublicense from Alnylam, the [**] Patent Rights (as defined in the
[**] Agreement) pursuant to the terms of this License Agreement.

          NOW, THEREFORE, in consideration of the respective covenants and
agreements contained herein, and for other valuable consideration, the receipt
and adequacy of which are hereby acknowledged, Alnylam and Biogen Idec agree as
follows:

                                    ARTICLE 1

                                   DEFINITIONS

          All capitalized terms used herein that are not otherwise defined
herein shall have the meaning given to such terms in the Collaboration
Agreement.

          "Blocking IP" shall mean any and all Patent Rights (other than those
licensed under this License Agreement or Patent Rights Controlled by Alnylam)
which, if claims covering subject matter of such Patent Rights issue, would
render the use, development, manufacture, sale, or other disposal of a Selected
RNAi Therapeutic which is a [**] Licensed Product unlawful in the absence of a
license to such Patent Rights from a Third Party.


                                       55

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          "[**] Agreement" shall have the meaning given to such term in the
Recitals.

          "[**] Field" shall mean the development of Selected RNAi Therapeutics
for the treatment of human disease (including by means of gene therapy).

          "[**] Licensed Product" shall mean a product or products which, or the
process of production of which, or the use of which falls within the scope of a
Valid Claim (as defined in the [**] Agreement) of the [**] Patent Rights.

          "[**] Patent Rights" shall have the meaning given to such term in the
[**] Agreement, which include the rights identified on Schedule A hereto.

                                    ARTICLE 2

                                  LICENSE GRANT

          2.1 LICENSES

          Subject to the terms and conditions of the Collaboration Agreement and
of Section 2.3 of this License Agreement, Alnylam hereby grants to Biogen Idec
and its Affiliates:

          (i) A co-exclusive (with Alnylam), worldwide, royalty-free right and
sublicense under [**] Patent Rights to Discover RNAi Therapeutics directed to
JCV that are [**] Licensed Products in the Field and to otherwise conduct the
Discovery Program in the Field, in each case to the extent necessary for Biogen
Idec to conduct its activities under the Annual Work Plan;

          (ii) A co-exclusive (with Alnylam), worldwide, royalty-free right and
sublicense under [**] Patent Rights to Develop Selected RNAi Therapeutics that
are [**] Licensed Products in the Field in the Territory, to the extent
necessary for Biogen Idec to conduct its activities under the Annual Work Plan;
and

          (iii) An exclusive, worldwide, royalty-free right and sublicense under
[**] Patent Rights to Commercialize Selected RNAi Therapeutics that are [**]
Licensed Products in the Field in the Territory.

          2.2 BIOGEN IDEC SUBLICENSE RIGHTS

          In addition to the requirements of the Collaboration Agreement
(including, but not limited to, Section 4.2 of the Collaboration Agreement),
sublicensing of Biogen Idec's rights under Section 2.1 will be permitted subject
to the following provisions: (i) Biogen Idec's Sublicensees shall have no right
to grant further sublicenses without Alnylam's written consent, which consent
shall not be unreasonably withheld or delayed; and (ii) Biogen Idec shall be
primarily liable for any failure by its Sublicensees to comply with, and Biogen
Idec guarantees to Alnylam the compliance by each of its Sublicensees


                                       56

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

with, all relevant restrictions, limitations and obligations in this License
Agreement. In the event of a conflict between the terms of the Collaboration
Agreement and this Section 2.2 with regard to any sublicense of the [**] Patent
Rights by Biogen Idec, this Section 2.2 will control.

          2.3 RESERVATION OF RIGHTS

          (i) For the sake of clarity, save to the extent necessary for the
Development and/or Commercialization of Selected RNAi Therapeutics which are
[**] Licensed Products in the [**] Field, neither Biogen Idec nor any Affiliate
is granted the right to make use of the [**] Patent Rights to research, develop,
use, keep, make, have made, sell and otherwise dispose or offer to dispose of
products:

               a)   for any diagnostic application;

               b)   as research tools or reagents;

               c)   for target validation; or

               d)   small molecule drug discovery

including the provision of services in relation thereto to Affiliates or Third
Parties (as defined in the [**] Agreement).

          (ii) Biogen Idec acknowledges and agrees that, pursuant to Section 2.3
of the [**] Agreement, [**] and [**] shall have the right to use, and [**] shall
have the right to consent to the use by academic research institutions
(including for the sake of clarity those in receipt of [**] funding) of, the
[**] Patent Rights in the Field for internal, or in collaboration with another
academic research institution, non-commercial, non-commercially sponsored
research. For the sake of clarity, [**]-funded Researchers (as defined in the
[**] Agreement) shall be permitted under the [**] Patent Rights to conduct
clinical trials of potential dsRNA therapeutic agents as part of their
[**]-funded academic research.

          (iii) Biogen Idec acknowledges and agrees that the grants by Alnylam
under [**] Patent Rights set forth in Sections 2.1 and 2.2 will be subject to
the terms and conditions of Sections 4.4, 6.4.2(a), 6.4.3 and 8.2.2(b)(i) of the
Collaboration Agreement as though [**] Patent Rights were included in Alnylam
Patent Rights and the [**] Agreement was an Existing Alnylam In-License.

                                    ARTICLE 3

                              FINANCIAL PROVISIONS

          3.1 UPFRONT CONSIDERATION


                                       57

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          In consideration of the rights granted to Biogen Idec under this
License Agreement as of the Effective Date, Biogen Idec shall pay, or cause to
be paid, to Alnylam [**] dollars ($[**]) within ten (10) Business Days following
the Effective Date.

          3.2 ROYALTIES

          Biogen Idec shall pay, or cause to be paid, to Alnylam royalties of
[**] per cent ([**]%) of Net Sales (as defined in the [**] Agreement) of Royalty
Licensed Products (as defined in the [**] Agreement) in the [**] Field.

          3.3 ROYALTY STACKING

          Provided always that the royalty payable under Section 3.2 of this
License Agreement to Alnylam shall not in any event be reduced below [**]
percent ([**]%), if at any time prior to or during the period for the payment of
royalties under this License Agreement in relation to any particular territory,
Biogen Idec (or its Affiliate as the case may be) or a Sublicensee elects in its
reasonable opinion to take a license from a Third Party to any Blocking IP to
develop, make, sell or otherwise dispose in the Field of Selected RNAi
Therapeutics that are [**] Licensed Products, the royalties set forth in Section
3.2 applicable to such Selected RNAi Therapeutic (as defined in the [**]
Agreement) shall be reduced by [**]% of the amount paid to such Third Party to
access said Blocking IP.

          3.4 CREDIT UNDER COLLABORATION AGREEMENT

          All payments made under the Collaboration Agreement shall be credited
against payments due under this Agreement such that no payments shall be due to
Alnylam under this Agreement that are in addition to the payments required to be
made by Biogen Idec to Alnylam under the Collaboration Agreement.

                                   ARTICLE IV

                         COMPLIANCE WITH [**] AGREEMENT

          Pursuant to the requirements of clause 2.4 of the [**] Agreement:

          4.1 TERMINATION

          The term and termination of this License Agreement shall be governed
by Article 8 of the Collaboration Agreement as though [**] Patent Rights were
included in Alnylam Patent Rights, provided that:


                                       58

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          (a) This License Agreement shall terminate automatically on the expiry
or termination for whatever reason of the [**] Agreement.

          (b) In the event of termination of the [**] Agreement pursuant to
Clause 10 of the [**] Agreement, [**] has agreed that it will enter into a
direct licensing arrangement with Biogen Idec on terms substantially similar to
those contained herein save that any license granted by [**] to Biogen Idec
shall be consistent with the terms of this License Agreement in relation to
field, territory, exclusivity, rights to sub-license and payment provisions.
However, in the event of termination of the [**] Agreement by Alnylam pursuant
to Clause 10.2 of such [**] Agreement, the provisions of the foregoing sentence
shall apply save that the granting of such license by [**] shall be subject to
[**]'s consent. Biogen Idec acknowledges and agrees that (i) [**] shall have no
obligation to enter into a direct licensing arrangement with Biogen Idec where
Biogen Idec is in default of its obligations under this License Agreement; (ii)
[**] shall not be expected to take any responsibility for any disputes between
Alnylam (or its Affiliate as the case may be) and Biogen Idec relating to the
terms of this License Agreement, and (iii) notwithstanding the foregoing
provisions of this Section 4.1(b), [**] shall not be obliged to enter into a
direct license with Biogen Idec in circumstances in which Biogen Idec reserves
any right to maintain a claim against [**] where such claim was previously
maintained against Alnylam (or its Affiliate as the case may be).

          4.2 BOOKS AND RECORDS

          Biogen Idec shall undertake to [**] directly to allow [**] the same
access to Biogen Idec's books and records as [**] has to Alnylam's books and
records under the [**] Agreement; and

          4.3 ASSIGNMENT

          (a) Subject to Section 4.3(b), neither Party shall without the prior
written consent of the other Party, which shall not be unreasonably withheld,
assign the benefit and/or burden of this License Agreement nor sub-contract any
of its obligations hereunder unless otherwise permitted by the terms hereof.

          (b) Either Party shall be entitled to assign the benefit and/or burden
of this License Agreement to any Affiliate or to its successor in connection
with any merger, consolidation or sale or other disposal of all or substantially
all of its assets and/or business to which this License Agreement relates.

          (c) Notwithstanding anything to the contrary contained in this
Agreement, if the Collaboration Agreement is assigned [**], Alnylam hereby
consents to


                                       59

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

the assignment of this Agreement [**] on the same terms and conditions as the
assignment of the Collaboration Agreement as set forth in such Section 10.2.2.

          4.4 SUB-LICENSES

          Any sub-license granted by Biogen Idec under this License Agreement
shall be subject to the terms of this Article IV and Sections 2.2 and 2.3 of
this License Agreement.

                                    ARTICLE V

                           OTHER TERMS AND CONDITIONS

          Except as explicitly set forth to the contrary in Articles I through
IV of this License Agreement, the grant of the [**] Patent Rights under Sections
2.1 and 2.2 shall be governed by the terms and conditions of the Collaboration
Agreement as though such [**] Patent Rights were included in Alnylam Patent
Rights. Alnylam has disclosed to Biogen Idec a redacted copy of the [**]
Agreement Contracts and all amendments thereto.

                  [Remainder of Page Intentionally Left Blank]


                                       60

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

          IN WITNESS WHEREOF, Alnylam and Biogen Idec have caused this License
Agreement to be duly executed by their authorized representatives, as of the
date first written above.

                                        ALNYLAM PHARMACEUTICALS, INC.


                                        By: /s/ John M. Maraganore
                                            ------------------------------------
                                        Name: John M. Maraganore
                                        Title: President & Chief Executive
                                               Officer


                                        BIOGEN IDEC, INC.


                                        By: /s/ James C. Mullen
                                            ------------------------------------
                                        Name: James C. Mullen
                                        Title: President & Chief Executive
                                               Officer


                                       61

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                                   SCHEDULE A

                               [**] PATENT RIGHTS

[**]

<TABLE>
<CAPTION>
                                                                   PUBLICATION
COUNTRY   TYPE   APPLICATION NO.   FILING DATE   PUBLICATION NO.       DATE      PATENT NO.   ISSUE DATE
-------   ----   ---------------   -----------   ---------------   -----------   ----------   ----------
<S>       <C>    <C>               <C>           <C>               <C>           <C>          <C>
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]                                         [**]         [**]
[**]      [**]                         [**]
[**]      [**]                         [**]
[**]      [**]         [**]            [**]
[**]      [**]         [**]            [**]
[**]      [**]         [**]            [**]                                         [**]         [**]
[**]      [**]         [**]            [**]                                         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]
[**]      [**]         [**]            [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
[**]      [**]         [**]            [**]            [**]            [**]         [**]         [**]
</TABLE>


                                       62

<PAGE>

                                                                    CONFIDENTIAL
                                                                  EXECUTION COPY

                                   SCHEDULE 9

                  EXCEPTIONS TO REPRESENTATIONS AND WARRANTIES

Section 9(a):

[**].

Section 9(b):

There are currently multi-party oppositions against the following European
patents included in Alnylam Patent Rights:

     (i)  Alnylam Europe AG European patent number 1144623. Amended claims for
          this patent were upheld in the oral phase of the opposition
          proceedings on June 21 and 22, 2006.

     (ii) Alnylam Europe AG European patent number 1214945.

     (iii) CRT European patent number 1230375.

A single party has also opposed Alnylam Europe AG Australian patent number
778474.

Section 9(c):

[**].


                                       63