$400,000,000 BOSTON PROPERTIES LIMITED PARTNERSHIP 3.75% EXCHANGEABLE SENIOR NOTES DUE2036 UNDERWRITING AGREEMENT

Exhibit 1.1

EXECUTION VERSION

$400,000,000

BOSTON PROPERTIES LIMITED PARTNERSHIP

3.75% EXCHANGEABLE SENIOR NOTES DUE 2036

UNDERWRITING AGREEMENT

March 31, 2006

EXECUTION VERSION

$400,000,000

BOSTON PROPERTIES LIMITED PARTNERSHIP

3.75% Exchangeable Senior Notes due 2036

Underwriting Agreement

March 31, 2006

Morgan Stanley & Co. Incorporated

1585 Broadway

New York, New York 10036

Ladies and Gentlemen:

Boston Properties Limited Partnership, a Delaware limited partnership (the “Company”), proposes to issue and sell to you, as Underwriter (the “Underwriter”), $400,000,000 principal amount of its 3.75% Exchangeable Senior Notes due 2036 (the “Firm Securities”) and has granted you an option to purchase up to an additional $60,000,000 principal amount of its 3.75% Exchangeable Senior Notes due 2036, solely to cover over-allotments (the “Additional Securities”). The Firm Securities and the Additional Securities are hereinafter collectively referred to as the “Securities”. The Securities will be issued pursuant to the Indenture dated as of December 13, 2002 between the Company and The Bank of New York Trust Company, N.A., as trustee (the “Trustee”), as supplemented by the Fifth Supplemental Indenture relating to the Securities to be dated as of March 31, 2006 between the Company and the Trustee (collectively, the “Indenture”). The Securities will be issued in book-entry form and will be issued to Cede & Co., as nominee of The Depository Trust Company (“DTC”), pursuant to a letter agreement, to be dated as of the Closing Date (as defined below) between the Company and DTC.

The Company has filed with the Securities and Exchange Commission (the “Commission”) a registration statement, including a prospectus (file no. 333-107792) on Form S-3, relating to securities (the “Shelf Securities”), including the Securities, to be issued from time to time by the Company. The registration statement as amended to the date of this Agreement, including the information (if any) deemed to be part of the registration statement at the time of effectiveness pursuant to Rule 430A or Rule 430B under the Securities Act of 1933, as amended (the “Securities Act”), is hereinafter referred to as the “Registration Statement,” and the related prospectus covering the Shelf Securities dated August 27, 2003 in the form first used to confirm sales of the Securities (or in the form first made available to the Underwriter by the Company to meet requests of purchasers pursuant to Rule 173 under the Securities Act) is hereinafter referred to as the “Basic Prospectus.” The Basic Prospectus, as supplemented by the prospectus supplement specifically relating to the Securities in the form first used to confirm sales of the Securities (or in the form first made available to the Underwriter by the Company to meet

requests of purchasers pursuant to Rule 173 under the Securities Act) is hereinafter referred to as the “Prospectus,” and the term “preliminary prospectus” means any preliminary form of the Prospectus. For purposes of this Agreement, (i) “free writing prospectus” has the meaning set forth in Rule 405 under the Securities Act, (ii) “Time of Sale Prospectus” means the preliminary prospectus together with the free writing prospectuses, each identified in Schedule I hereto, and (iii) “broadly available road show” means a “bona fide electronic road show” as defined in Rule 433(h)(5) under the Securities Act that has been made available without restriction to any person. As used herein, the terms “Registration Statement, “Basic Prospectus,” “preliminary prospectus,” “Time of Sale Prospectus” and Prospectus shall include the documents, if any, incorporated by reference therein. The terms “supplement,” “amendment,” and amend” as used herein with respect to the Registration Statement, the Basic Prospectus, the Time of Sale Prospectus, any preliminary prospectus or free writing prospectus shall include all documents subsequently filed by the Company with the Commission pursuant to the Securities Exchange Act of 1934, as amended (the “Exchange Act”), that are deemed to be incorporated by reference therein.

Holders of the Securities will be entitled to the benefits of a Registration Rights Agreement, to be dated as of the Closing Date, providing for the registration under the Securities Act of any shares of Boston Properties, Inc. common stock that have not been registered under the Securities Act delivered upon exchange of the Securities.

1. Representations and Warranties. The Company represents and warrants to the Underwriter as of the date hereof and as of the Closing Date (as defined herein) and each Option Closing Date (as defined herein) and agrees with the Underwriter that:

(a) The Registration Statement has become effective; no stop order suspending the effectiveness of the Registration Statement is in effect, and no proceedings for such purpose are pending before or threatened by the Commission.

(b) (i) Each document, if any, filed or to be filed pursuant to the Exchange Act and incorporated by reference in the Time of Sale Prospectus or the Prospectus complied or will comply when so filed in all material respects with the Exchange Act and the applicable rules and regulations of the Commission thereunder, (ii) each part of the Registration Statement, when such part became effective, did not contain, and each such part, as amended or supplemented, if applicable, will not contain any untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading, (iii) the Registration Statement as of the date hereof does not contain any untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading, (iv) the Registration Statement and the Prospectus comply, and as amended or supplemented, if applicable, will comply in all material respects with the Securities Act and the applicable rules and regulations of the Commission thereunder, (v) the Time of Sale Prospectus does not, and at the time of each sale of the Securities in connection with the offering when the Prospectus is not yet available to prospective purchasers and at the Closing Date (as defined in Section 4), the Time of Sale Prospectus, as then amended or supplemented by the Company, if applicable, will not, contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading, (vi) each broadly available road show, if any, when considered together with the Time of Sale Prospectus, does not contain any

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untrue statement of a material fact or omit to state a material fact necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading and (vii) the Prospectus does not contain and, as amended or supplemented, if applicable, will not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading, except that the representations and warranties set forth in this paragraph do not apply to (A) statements or omissions in the Registration Statement, the Time of Sale Prospectus or the Prospectus based upon information relating to the Underwriter furnished to the Company in writing by the Underwriter expressly for use therein or (B) that part of the Registration Statement that constitutes the Statement of Eligibility (Form T-1) under the Trust Indenture Act of 1939, as amended (the Trust Indenture Act”), of the Trustee.

(c) The Company is not an “ineligible issuer” in connection with the offering pursuant to Rule 164, 405 and 433 under the Securities Act. Any free writing prospectus that the Company is required to file pursuant to Rule 433(d) under the Securities Act has been, or will be, filed with the Commission in accordance with the requirements of the Securities Act and the applicable rules and regulations of the Commission thereunder. Each free writing prospectus that the Company has filed, or is required to file, pursuant to Rule 433(d) under the Securities Act or that was prepared by or behalf of or used or referred to by the Company complies or will comply in all material respects with the requirements of the Securities Act and the applicable rules and regulations of the Commission thereunder. Except for the free writing prospectuses, if any, identified in Schedule I hereto, and electronic road shows each furnished to you before first use, the Company has not prepared, used or referred to, and will not, without your prior consent, prepare, use or refer to, any free writing prospectus.

(d) The consolidated financial statements incorporated by reference in the Registration Statement, the Time of Sale Prospectus and the Prospectus, together with the related notes, present fairly the financial position of the Company at the dates indicated or for the periods specified, as the case may be; said financial statements have been prepared in conformity with generally accepted accounting principles of the United States of America (“GAAP”) applied on a consistent basis throughout the periods involved. The selected financial data included or incorporated by reference in the Registration Statement, the Time of Sale Prospectus and the Prospectus present fairly the information shown therein and have been compiled on a basis consistent with that of the audited financial statements incorporated by reference in the Prospectus.

(e) Since December 31, 2005, except as described in the Time of Sale Prospectus or in documents incorporated by reference therein, (i) there has been no material adverse change in the condition, financial or otherwise, or in the earnings, business affairs or business prospects of the Company and its Subsidiaries (as hereinafter defined) considered as one enterprise, whether or not arising in the ordinary course of business (a “Material Adverse Effect”), (ii) no material casualty loss or material condemnation or other material adverse event with respect to any of the commercial real estate properties owned by the Company as of the date of this Agreement (the “Properties”) has occurred, and (iii) there have been no transactions entered into by the Company or any of its Subsidiaries, other than those in the ordinary course of business, which are material with respect to the Company and the Subsidiaries considered as one enterprise.

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(f) The Company has been duly organized and is validly existing as a limited partnership in good standing under the laws of the State of Delaware and has partnership power and authority to own, lease and operate its properties and to conduct its business as described in the Prospectus and to enter into and perform its obligations under this Agreement; and the Company is duly qualified as a foreign partnership to transact business and is in good standing in each other jurisdiction in which such qualification is required, whether by reason of the ownership or leasing of property or the conduct of business, except where the failure so to qualify or to be in good standing would not result in a Material Adverse Effect.

(g) Each of the subsidiaries of the Company set forth on Schedule II hereto (each a “Subsidiary” and, collectively, the “Subsidiaries”), has been duly organized and is validly existing as a general or limited partnership, limited liability company or corporation, as the case may be, in good standing (in the case of corporations and limited partnerships) under the laws of the jurisdiction of its organization, has partnership or corporate power and authority, as the case may be, to own, lease and operate its properties and to conduct its business as described in the Prospectus and is duly qualified as a foreign entity to transact business and is in good standing in each jurisdiction in which such qualification is required, whether by reason of the ownership or leasing of property or the conduct of business, except where the failure so to qualify or to be in good standing would not result in a Material Adverse Effect. The Subsidiaries collectively own not less than 90% of the consolidated assets of the Company and its subsidiaries as of December 31, 2005. All of the issued and outstanding capital stock of each of the Subsidiaries that is a corporation has been duly authorized and validly issued, is fully paid and non-assessable, and all of the partnership interests in each Subsidiary that is a partnership are validly issued and fully paid. Except as otherwise disclosed in Schedule III hereto or in the Prospectus, all such shares and interests, as the case may be, are owned by the Company, directly or through Subsidiaries, free and clear of any security interest, mortgage, pledge, lien, encumbrance, claim or equity, except where such security interest, mortgage, pledge, lien, encumbrance, claim or equity would not reasonably be expected to result in a Material Adverse Effect. None of the outstanding shares of capital stock or partnership interests of any Subsidiary was issued in violation of the preemptive or similar rights of any securityholder of such Subsidiary.

(h) The partnership interests of the Company have been duly authorized and validly issued and are fully paid; none of the outstanding partnership interests of the Company was issued in violation of the preemptive or other similar rights of any securityholder of the Company.

(i) The Company has full right, power and authority to execute and deliver this Agreement, the Securities and the Indenture (collectively, the “Transaction Documents”) and to perform its obligations hereunder and thereunder; and all action required to be taken for the due and proper authorization, execution and delivery of each of the Transaction Documents and the consummation of the transactions contemplated thereby has been duly and validly taken.

(j) The Indenture has been duly qualified under the Trust Indenture Act and has been duly authorized, executed and delivered by, and is a valid and binding agreement of, the Company, enforceable in accordance with its terms, subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally and equitable principles of general applicability (the “Enforceability Exceptions”).

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(k) The Securities have been duly authorized by the Company and, when duly executed, authenticated, issued and delivered as provided in the Indenture and paid for as provided herein, will be duly and validly issued and outstanding and will constitute valid and legally binding obligations of the Company enforceable against the Company in accordance with their terms, subject to the Enforceability Exceptions, and will be entitled to the benefits of the Indenture.

(l) Each Transaction Document conforms in all material respects to the description thereof contained in the Prospectus.

(m) Neither the Company nor any of its Subsidiaries is in violation of its organizational documents or in default in the performance or observance of any obligation, agreement, covenant or condition contained in any contract, indenture, mortgage, deed of trust, loan or credit agreement, note, lease or other agreement or instrument to which the Company or any of its Subsidiaries is a party or by which it or any of them may be bound, or to which any of the property or assets of the Company or any Subsidiary is subject (collectively, “Agreements and Instruments”) except for such defaults that would not result in a Material Adverse Effect. The execution, delivery and performance of this Agreement and the consummation of the transactions contemplated in this Agreement and the Prospectus (including the issuance and sale of the Securities and the use of the proceeds from the sale of the Securities as described in the Prospectus under the caption “Use of Proceeds”) and compliance by the Company with its obligations under this Agreement (except as contemplated by the Prospectus) do not and will not, whether with or without the giving of notice or passage of time or both, conflict with or constitute a breach of, or default or Repayment Event (as defined below) under, or result in the creation or imposition of any lien, charge or encumbrance upon any of the commercial real estate properties owned by the Company as of the date of this Agreement (the “Properties”) or any other property or assets of the Company or any Subsidiary pursuant to, the Agreements and Instruments or violation of any applicable law, statute, rule, regulation, judgment, order, writ or decree of any government, government instrumentality or court, domestic or foreign, having jurisdiction over the Company or any Subsidiary or any of their assets, properties or operations (except for such conflicts, breaches or defaults or liens, charges, encumbrances or violations that would not result in a Material Adverse Effect), nor will such action result in any violation of the provisions of the organizational documents of the Company or any Subsidiary. As used herein, the term “Repayment Event” means any event or condition which gives the holder of any note, debenture or other evidence of indebtedness (or any person acting on such holder’s behalf) the right to require the repurchase, redemption or repayment of all or a portion of such indebtedness by the Company or any Subsidiary.

(n) No filing with, or authorization, approval, consent, license, order, registration, qualification or decree of, any court or governmental authority or agency is necessary or required for the performance by the Company of its obligations hereunder, in connection with the offering, issuance or sale of the Securities under this Agreement or the consummation of the transactions contemplated by this Agreement, except such as have been already obtained or as may be required under the Securities Act and foreign or state securities or blue sky laws.

(o) Except as described in the Time of Sale Prospectus, there is no action, suit or proceeding before or brought by any court or governmental agency or body, domestic or foreign,

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now pending, or, to the knowledge of the Company, threatened, against or affecting the Company or any Subsidiary, which might reasonably be expected, if determined adversely to the Company or any Subsidiary to result in a Material Adverse Effect, or which might reasonably be expected to materially and adversely affect the Properties or assets thereof or the consummation of the transactions contemplated in this Agreement or the performance by the parties of their obligations hereunder.

(p) (i) The Company and the Subsidiaries have either good and marketable title in fee simple or good and marketable leasehold title, as applicable, to all of the Properties and good and marketable title to all other real properties owned by them, in each case, free and clear of all mortgages, pledges, liens, security interests, claims, restrictions or encumbrances of any kind except such as (a) are described in the Time of Sale Prospectus or (b) do not, singly or in the aggregate, materially affect the value of such property and do not interfere with the use made and proposed to be made of such property by the Company or any of its Subsidiaries; (ii) all mortgages, pledges, liens, security interests, claims, restrictions or encumbrances on or affecting the properties and assets of the Company or any of the Subsidiaries that are required to be disclosed in the Time of Sale Prospectus are disclosed therein; (iii) the Company does not know of any violation of any municipal, state or federal law, rule or regulation (including those pertaining to environmental matters) concerning the Properties or any part thereof which would have a Material Adverse Effect; (iv) each of the Properties complies with all applicable zoning laws, ordinances, regulations and deed restrictions or other covenants in all material respects and, if and to the extent there is a failure to comply, such failure does not result in a Material Adverse Effect and will not result in a forfeiture or reversion of title; (v) none of the Company or any Subsidiary has received from any governmental authority any written notice of any condemnation of or zoning change affecting the Properties or any part thereof which could have a Material Adverse Effect, and none of the Company or any Subsidiary knows of any such condemnation or zoning change which is threatened and which if consummated would have a Material Adverse Effect; and (vi) no lessee of any portion of any of the Properties is in default under any of the leases governing such Properties and there is no event which, but for the passage of time or the giving of notice or both, would constitute a default under any of such leases, except such defaults that would not have a Material Adverse Effect.

(q) Except as set forth in the Time of Sale Prospectus, the mortgages and deeds of trust encumbering the properties and assets described in the Time of Sale Prospectus are not convertible and neither the Company, any of its Subsidiaries, or any person affiliated therewith holds a participating interest therein, and such mortgages and deeds of trust are not cross-defaulted or cross-collateralized to any property not owned directly or indirectly by the Company or any of its Subsidiaries.

(r) The Company and its Subsidiaries possess such permits, licenses, approvals, consents and other authorizations (collectively, “Governmental Licenses”) issued by the appropriate federal, state, local or foreign regulatory agencies or bodies necessary to conduct the business now operated by them; the Company and its Subsidiaries are in compliance with the terms and conditions of all such Governmental Licenses, except where the failure so to comply would not, singly or in the aggregate, have a Material Adverse Effect; all of the Governmental Licenses are valid and in full force and effect, except when the invalidity of such Governmental Licenses or the failure of such Governmental Licenses to be in full force and effect would not

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have a Material Adverse Effect; and neither the Company nor any of its Subsidiaries has received any written notice of proceedings relating to the revocation or modification of any such Governmental Licenses which, singly or in the aggregate, if the subject of an unfavorable decision, ruling or finding, would result in a Material Adverse Effect.

(s) No material labor dispute with the employees of the Company or any Subsidiary exists or, to the knowledge of the Company, is imminent.

(t) Except as otherwise disclosed in the Time of Sale Prospectus or except as would not, singly or in the aggregate, have a Material Adverse Effect, (i) to the best knowledge of the Company, the Company and its Subsidiaries have been and are in compliance with applicable Environmental Statutes; (ii) to the best knowledge of the Company, neither the Company, any of its Subsidiaries, nor any other owners of the property at any time or any other party has at any time released (as such term is defined in Section 101(22) of CERCLA (as hereinafter defined)) or otherwise disposed of Hazardous Materials (as hereinafter defined) on, to or from the Properties; (iii) the Company does not intend to use the Properties or any subsequently acquired properties, other than in compliance with applicable Environmental Statutes (as hereinafter defined); (iv) neither the Company nor any of its Subsidiaries knows of any seepage, leak, discharge, release, emission, spill, or dumping of Hazardous Materials into waters (including, but not limited, to groundwater and surface water) on, beneath or adjacent to the Properties or onto lands from which Hazardous Materials might seep, flow or drain into such waters; (v) neither the Company nor any of its Subsidiaries has received any notice of, or has any knowledge of any occurrence or circumstance which, with notice or passage of time or both, would give rise to a claim under or pursuant to any Environmental Statute with respect to the Properties or the assets described in the Time of Sale Prospectus or arising out of the conduct of the Company or its Subsidiaries; (vi) neither the Properties nor any other land owned by the Company or any of its Subsidiaries is included or, to the best of the Company’s knowledge, proposed for inclusion on the National Priorities List issued pursuant to CERCLA by the United States Environmental Protection Agency (the “EPA”) or to the best of the Company’s knowledge, proposed for inclusion on any similar list or inventory issued pursuant to any other Environmental Statute or issued by any other Governmental Authority (as hereinafter defined).

As used herein, “Hazardous Material” shall include, without limitation any flammable explosives, radioactive materials, hazardous materials, hazardous wastes, toxic substances, or related materials, asbestos or any hazardous material as defined by any federal, state or local environmental law, ordinance, rule or regulation including, without limitation, the Comprehensive Environmental Response, Compensation, and Liability Act of 1980, as amended, 42 U.S.C. §§ 9601-9675 (“CERCLA”), the Hazardous Materials Transportation Act, as amended, 49 U.S.C. §§ 1801-1819, the Resource Conservation and Recovery Act, as amended, 42 U.S.C. §§ 6901-K, the Emergency Planning and Community Right-to-Know Act of 1986, 42 U.S.C. §§ 11001-11050, the Toxic Substances Control Act, 15 U.S.C. §§ 2601-2671, the Federal Insecticide, Fungicide and Rodenticide Act, 7 U.S.C. §§ 136-136y, the Clean Air Act, 42 U.S.C. §§ 7401-7642, the Clean Water Act (Federal Water Pollution Control Act), 33 U.S.C. §§ 1251-1387, the Safe Drinking Water Act, 42 U.S.C. §§ 300f-300j-26, and the Occupational Safety and Health Act, 29 U.S.C. §§ 651-678, as any of the above statutes may be amended from time to time, and in the regulations promulgated pursuant to each of the foregoing (including environmental statutes not specifically defined herein) (individually, an “Environmental Statute”

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and collectively “Environmental Statutes”) or by any federal, state or local governmental authority having or claiming jurisdiction over the properties and assets described in the Time of Sale Prospectus (a “Governmental Authority”).

(u) The Company and each of the Subsidiaries is insured by insurers of recognized financial responsibility against such losses and risks and in such amounts as are prudent and customary in the businesses in which they will be engaged; and neither the Company nor any of the Subsidiaries has any reason to believe that any of them will not be able to renew its existing insurance coverage as and when such coverage expires or to obtain similar coverage from similar insurers as may be necessary to continue its business, assuming that such coverage continues to be available on commercially reasonable terms at the time.

(v) The Company and each of the Subsidiaries has filed all material foreign, federal, state and local tax returns that are required to be filed or have requested extensions thereof (except in any case in which the failure so to file would not have a Material Adverse Effect) and has paid all taxes required to be paid by it and any other assessment, fine or penalty levied against it, to the extent that any of the foregoing is due and payable, except for any such assessment, fine or penalty that is currently being contested in good faith or as described in or contemplated by the Prospectus.

(w) No forward-looking statement (within the meaning of Section 27A of the Securities Act and Section 21E of the Exchange Act) contained in the Prospectus has been made or reaffirmed without a reasonable basis or has been disclosed other than in good faith.

(x) PricewaterhouseCoopers LLP, who have certified the financial statements and supporting schedules, if any, of the Company and its Subsidiaries included in the Prospectus, are independent registered public accountants with respect to the Company and its Subsidiaries within the meaning of Rule 101 of the Code of Professional Conduct of the American Institute of Certified Public Accountants and its interpretations and rulings thereunder.

(y) The Company is not, and upon the issuance and sale of the Securities as herein contemplated and the application of the net proceeds therefrom as described in the Prospectus will not be, an “investment company” as such term is defined in the Investment Company Act of 1940, as amended (the “1940 Act”).

(z) Each preliminary prospectus filed as part of the registration statement as originally filed or as part of any amendment thereto, or filed pursuant to Rule 424 under the Securities Act, complied when so filed in all material respects with the Securities Act and the applicable rules and regulations of the Commission thereunder.

(aa) None of the Company or any of its Subsidiaries is a party to any contract, agreement or understanding with any person (other than this Agreement) that would give rise to a valid claim against any of them or the Underwriter for a brokerage commission, finder’s fee or like payment in connection with the offering and sale of the Securities.

(bb) Neither the Company nor any of its partners, officers or affiliates has taken or will take, directly or indirectly, any action designed to or that could reasonably be expected, under the

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Exchange Act or otherwise, to cause or result in any stabilization or manipulation of the price of the Securities.

2. Agreements to Sell and Purchase. The Company hereby agrees to sell to the Underwriter, upon the basis of the representations and warranties herein contained , but subject to the conditions hereinafter stated, agrees to purchase the Securities from the Company at the purchase price of 99.914620% of the principal amount thereof, plus accrued interest, if any, to the Closing Date.

On the basis of the representations and warranties contained in this Agreement, and subject to its terms and conditions, the Company agrees to sell to the Underwriter the Additional Securities, and the Underwriter shall have the right to purchase up to $60,000,000 aggregate principal amount of Additional Securities at the purchase price of 99.914620% of the principal amount thereof, plus accrued interest, if any, to the applicable Option Closing Date. You may exercise this right in whole or from time to time in part by giving written notice not later than 30 days after the date of this Agreement. Any exercise notice shall specify the number of Additional Securities to be purchased by you and the date on which such Additional Securities are to be purchased (each, an “Option Closing Date”). Each purchase date must be at least one business day after the written notice is given and may not be earlier than the closing date for the Firm Securities nor later than ten business days after the date of such notice.

3. Public Offering. The Company is advised that the Underwriter proposes to make a public offering of the Securities as soon after the Registration Statement and this Agreement have become effective as in its judgment is advisable. The Company is further advised that the Securities are to be offered to the public upon the terms set forth in the Prospectus.

4. Payment and Delivery. Payment for the Securities shall be made to the Company by wire transfer of immediately available funds on April 6, 2006 at 10:00 a.m., or at such other time on the same or such other date, not later than the fifth business day thereafter, as may be designated by you in writing. The time and date of such payment are hereinafter referred to as the “Closing Date.”

Payment for any Additional Securities shall be made to the Company by wire transfer of immediately available funds against delivery of such Additional Securities for the account of the Underwriter at 10:00 a.m., New York City time, on the Option Closing Date or at such other time on the same or on such other date, in any event not later than the fifth business day thereafter, as may be designated in writing by you.

Payment for the Securities shall be made against delivery to you on the Closing Date or the applicable Option Closing Date, as the case may be, of the Securities registered in such names and in such denominations as you shall request in writing not less than one full business day prior to the Closing Date or the applicable Option Closing Date, as the case may be, with any transfer taxes payable in connection with the transfer of the Securities to the Underwriter duly paid.

5. Further Agreements of the Company. The Company covenants and agrees with Underwriter:

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(a) The Company will comply with the requirements of Rule 430A under the Securities Act or Rule 430B under the Securities Act, as applicable, and will notify the Underwriter promptly, and confirm the notice in writing, (i) when any post-effective amendment to the Registration Statement shall become effective, or any supplement to the Prospectus or any amended Prospectus shall have been filed, (ii) of the receipt of any comments from the Commission, (iii) of any request by the Commission for any amendment to the Registration Statement or any amendment or supplement to the Prospectus or for additional information, and (iv) of the issuance by the Commission of any stop order suspending the effectiveness of the Registration Statement or of any order by any governmental or regulatory authority preventing or suspending the use of any preliminary prospectus, or of the suspension of the qualification of the Securities for offering or sale in any jurisdiction, or of the initiation or threatening of any proceedings for any of such purposes. The Company will promptly effect the filings necessary pursuant to Rule 424(b) under the Securities Act and will take such steps as it deems necessary to ascertain promptly whether the form of prospectus transmitted for filing under Rule 424(b) under the Securities Act was received for filing by the Commission and, in the event that it was not, it will promptly file such prospectus. The Company will make every reasonable effort to prevent the issuance of any such order and, if any such order is issued, to obtain the lifting thereof at the earliest possible moment.

(b) Before amending or supplementing the Registration Statement, the Time of Sale Prospectus or the Prospectus during the period mentioned in Section 5(f) below, to furnish to you a copy of each such proposed amendment or supplement and not to file any such proposed amendment or supplement to which you reasonably object.

(c) During the period mentioned in Section 5(f) below, to furnish to you a copy of each proposed free writing prospectus to be prepared by or on behalf of, used by, or referred to by the Company and not to use or refer to any proposed free writing prospectus to which you reasonably object.

(d) Not to take any action that would result in the Underwriter or the Company being required to file with the Commission pursuant to Rule 433(d) under the Securities Act a free writing prospectus prepared by or on behalf of the Underwriter that the Underwriter otherwise would not have been required to file thereunder.

(e) If the Time of Sale Prospectus is being used to solicit offers to buy the Securities at a time when the Prospectus is not yet available to prospective purchaser and any event shall occur or condition exist as a result of which it is necessary to amend or supplement the Time of Sale Prospectus in order to make the statements therein, in light of the circumstances, not misleading, or if any event shall occur or condition exist as a result of which the Time of Sale Prospectus conflicts with the information contained in the Registration Statement then on file, or if it is necessary to amend or supplement the Time of Sale Prospectus to comply with applicable law forthwith to prepare, file with the Commission and furnish, at its own expense, to the Underwriter and to any dealer upon request, either amendments or supplements to the Time of Sale Prospectus so that the statements in the Time of Sale Prospectus as so amended or supplemented will not, in light of the circumstances when delivered to a prospective purchaser, be misleading or so that the Time of Sale Prospectus, as amended or supplemented, will no

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longer conflict with the Registration Statement, or so that the Time of Sale Prospectus, as amended or supplemented, will comply with applicable law.

(f) If, during such period after the first date of the public offering of the Securities as the Prospectus (or in lieu thereof the notice referred to in Rule 173(a) under the Securities Act) is required by law to be delivered in connection with sales by the Underwriter or a dealer, any event shall occur or condition exist as a result of which it is necessary to amend or supplement the Prospectus in order to make the statements therein, in the light of the circumstances when the Prospectus (or in lieu thereof the notice referred to in Rule 173(a) under the Securities Act) is delivered to a purchaser, not misleading, or if it is necessary to amend or supplement the Prospectus to comply with applicable law, forthwith to prepare, file with the Commission and furnish, at its own expense, to the Underwriter and to the dealers (whose names and addresses you will furnish to the Company) to which Securities may have been sold by you on behalf of the Underwriter and to any other dealers upon request, either amendments or supplements to the Prospectus so that the statements in the Prospectus as so amended or supplemented will not, in the light of the circumstances when the Prospectus (or in lieu thereof the notice referred to in Rule 173(a) under the Securities Act) is delivered to a purchaser, be misleading or so that the Prospectus, as amended or supplemented, will comply with applicable law.

(g) To endeavor to qualify the Securities for offer and sale under the securities or Blue Sky laws of such jurisdictions as you shall reasonably request; provided that in no event shall the Company be obligated to qualify to do business in any jurisdiction where it is not now so qualified or take any action that would subject it to general service of process suits, other than those arising out of the offering or sale of the securities as contemplated by this Agreement and the Prospectus, in any jurisdiction where it is not now subject.

(h) To make generally available to the Company’s security holders and to you as soon as practicable an earning statement covering a period of at least twelve months beginning with the first fiscal quarter of the Company occurring after the date of this Agreement which shall satisfy the provisions of Section 11(a) of the Securities Act and the rules and regulations of the Commission thereunder.

(i) The Company will apply the net proceeds from the sale of the Securities as described in the Prospectus under the heading “Use of Proceeds.”

(j) The Company will cooperate with the Underwriter in arranging for the Securities to be eligible for clearance and settlement through DTC.

(k) The Company will file promptly all reports and any definitive proxy or information statements, if any, required to be filed by the Company with the Commission pursuant to Section 13(a), 13(c), 14 or 15(d) of the Exchange Act.

6. Conditions of the Underwriter’s Obligations. The obligation of the Underwriter to purchase Securities on the Closing Date and each Option Closing Date as provided herein is subject to the performance by the Company of its respective covenants and other obligations hereunder and to the following additional conditions:

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(a) The representations and warranties of the Company contained herein shall be true and correct on the date hereof and on and as of the Closing Date and each Option Closing Date; and the statements of the Company and its respective officers made in any certificates delivered pursuant to this Agreement shall be true and correct on and as of the Closing Date and each Option Closing Date.

(b) Subsequent to the execution and delivery of this Agreement, (i) no downgrading shall have occurred in the rating assigned to the Securities or any other debt securities or preferred stock issued or guaranteed by the Company by any “nationally recognized statistical rating organization”, as such term is defined by the Commission for purposes of Rule 436(g)(2) under the Securities Act; and (ii) no such organization shall have publicly announced that it has under surveillance or review, or has changed its outlook with respect to, its rating of the Securities or of any other debt securities or preferred stock issued or guaranteed by the Company or any of its Subsidiaries (other than an announcement with positive implications of a possible upgrading).

(c) Subsequent to the execution and delivery of this Agreement, no event or condition of a type described in Section 1(e) hereof shall have occurred or shall exist, which event or condition is not described in the Prospectus (excluding any amendment or supplement thereto) and the effect of which in the judgment of the Underwriter makes it impracticable or inadvisable to proceed with the offering, sale or delivery of the Securities on the terms and in the manner contemplated by this Agreement and the Prospectus.

(d) The Underwriter shall have received on and as of the Closing Date and each Option Closing Date a certificate of an executive officer of the Company who has specific knowledge of the Company’s financial matters and is satisfactory to the Underwriter (i) confirming that such officer has carefully reviewed the Prospectus and, to the best knowledge of such officer, (ii) the representations and warranties of the Company in this Agreement are true and correct and that the Company has complied with all agreements and satisfied all conditions on its part to be performed or satisfied hereunder at or prior to the Closing Date and each Option Closing Date and (iii) confirming no event having the effect set forth in paragraphs (b) and (c) above shall have occurred.

(e) On the date of this Agreement and on the Closing Date and each Option Closing Date, PricewaterhouseCoopers LLP shall have furnished to the Underwriter, at the request of the Company, letters, dated the respective dates of delivery thereof and addressed to the Underwriter, in form and substance reasonably satisfactory to the Underwriter, containing statements and information of the type customarily included in accountants’ “comfort letters” to underwriters with respect to the financial statements and certain financial information contained or incorporated by reference in the Prospectus; provided that the letter delivered on the Closing Date and each Option Closing Date shall use a “cut-off” date no more than three business days prior to the Closing Date or such Option Closing Date, as the case may be.

(f) Goodwin Procter LLP, counsel for the Company, shall have furnished to the Underwriter, at the request of the Company, their written opinion, dated the Closing Date and each Option Closing Date and addressed to the Underwriter, in form and substance reasonably satisfactory to the Underwriter, to the effect set forth in Annex A hereto.

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(g) Frank Burt, Esq., General Counsel for the Company, shall have furnished to the Underwriter, at the request of the Company, his written opinion, dated the Closing Date and each Option Closing Date and addressed to the Underwriter, in form and substance reasonably satisfactory to the Underwriter, to the effect set forth in Annex B hereto.

(h) The Underwriter shall have received on and as of the Closing Date and each Option Closing Date opinions of Skadden, Arps, Slate, Meagher & Flom LLP and Davis Polk & Wardwell LLP, counsels for the Underwriter, with respect to such matters as the Underwriter may reasonably request, and such counsel shall have received such documents and information as they may reasonably request to enable them to pass upon such matters.

(i) No action shall have been taken and no statute, rule, regulation or order shall have been enacted, adopted or issued by any federal, state or foreign governmental or regulatory authority that would, as of the Closing Date and each Option Closing Date, prevent the issuance or sale of the Securities; and no injunction or order of any federal, state or foreign court shall have been issued that would, as of the Closing Date and each Option Closing Date, prevent the issuance or sale of the Securities.

(j) The Underwriter shall have received on and as of the Closing Date and each Option Closing Date satisfactory evidence of the good standing of the Company and each of the Subsidiaries which owns material assets, each of which is set forth on Schedule IV hereto, in their respective jurisdictions of organization and their good standing in such other jurisdictions as the Underwriter may reasonably request, in each case in writing or any standard form of telecommunication, from the appropriate governmental authorities of such jurisdictions.

(k) The Securities shall be eligible for clearance and settlement through DTC.

(l) On or prior to the Closing Date and each Option Closing Date, the Company shall have furnished to the Underwriter such further certificates and documents as the Underwriter may reasonably require for the purpose of consummating the transactions contemplated hereby.

(m) During the period beginning on the date hereof and continuing to and including the Closing Date, not to offer, sell or otherwise dispose of any debt securities of the Company or warrants to purchase or otherwise acquire debt securities of the Company substantially similar to the securities (other than (i) the Securities, (ii) commercial paper issued in the ordinary course of business or (iii) securities or warrants permitted with the prior written consent of the Underwriter).

All opinions, letters, certificates and evidence mentioned above or elsewhere in this Agreement shall be deemed to be in compliance with the provisions hereof only if they are in form and substance reasonably satisfactory to counsel for the Underwriter.

7. Covenants of the Underwriter. The Underwriter covenants with the Company not to take any action that would result in the Company being required to file with the Commission under Rule 433(d) a free writing prospectus prepared by or on behalf of the Underwriter that otherwise would not be filed by the Company thereunder, but for the action of the Underwriter and to furnish to the Company a copy of each proposed free writing prospectus to be used by

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you, and not to use any proposed free writing prospectus to which the Company reasonably objects.

8. Indemnification and Contribution.

(a) The Company agrees to indemnify and hold harmless the Underwriter, its affiliates, directors and officers and each person, if any, who controls such Underwriter within the meaning of Section 15 of the Securities Act or Section 20 of the Exchange Act, from and against any and all losses, claims, damages and liabilities (including, without limitation, legal fees and other reasonable expenses incurred in connection with any suit, action or proceeding or any claim asserted, as such fees and expenses are incurred), that arise out of, or are based upon, any untrue statement or alleged untrue statement of a material fact contained in the Registration Statement or any amendment thereof, any preliminary prospectus, the Time of Sale Prospectus, any issuer free writing prospectus as defined in Rule 433(h) under the Securities Act, any Company information that the Company has filed, or is required to file, pursuant to Rule 433(d) under the Securities Act or the Prospectus or any amendment or supplement thereto, or any omission or alleged omission to state therein a material fact necessary in order to make the statements therein, not misleading, except insofar as such losses, claims, damages or liabilities arise out of, or are based upon, any untrue statement or omission or alleged untrue statement or omission made in reliance upon and in conformity with any information relating to the Underwriter furnished to the Company in writing by the Underwriter expressly for use therein. The foregoing indemnity shall not apply to any issuer free writing prospectus used by the Underwriter in violation of Section 7 hereof.

(b) The Underwriter agrees to indemnify and hold harmless the Company, its affiliates, directors and officers, and each person, if any, who controls the Company within the meaning of Section 15 of the Securities Act or Section 20 of the Exchange Act to the same extent as the indemnity set forth in paragraph (a) above, but only with respect to any losses, claims, damages or liabilities that arise out of, or are based upon, any untrue statement or omission or alleged untrue statement or omission made in reliance upon and in conformity with any information relating to the Underwriter furnished to the Company in writing by the Underwriter expressly for use in the Registration Statement, any preliminary prospectus, the Time of Sale Prospectus, any issuer free writing prospectus or the Prospectus or any amendment or supplement thereto.

(c) Notice and Procedures. If any suit, action, proceeding (including any governmental or regulatory investigation), claim or demand shall be brought or asserted against any person in respect of which indemnification may be sought pursuant to either paragraph (a) or (b) above, such person (the “Indemnified Person”) shall promptly notify the person against whom such indemnification may be sought (the “Indemnifying Person”) in writing; provided that the failure to notify the Indemnifying Person shall not relieve it from any liability that it may have under this Section except to the extent that it has been materially prejudiced (through the forfeiture of substantive rights or defenses) by such failure; and provided, further, that the failure to notify the Indemnifying Person shall not relieve it from any liability that it may have to an Indemnified Person otherwise than under this Section. If any such proceeding shall be brought or asserted against an Indemnified Person and it shall have notified the Indemnifying Person thereof, the Indemnifying Person shall retain counsel reasonably satisfactory to the Indemnified Person to represent the

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Indemnified Person and any others entitled to indemnification pursuant to this Section that the Indemnifying Person may designate in such proceeding and shall pay the fees and expenses of such counsel related to such proceeding, as incurred. In any such proceeding, any Indemnified Person shall have the right to retain its own counsel, but the fees and expenses of such counsel shall be at the expense of such Indemnified Person unless (i) the Indemnifying Person and the Indemnified Person shall have mutually agreed to the contrary; (ii) the Indemnifying Person has failed within a reasonable time to retain counsel reasonably satisfactory to the Indemnified Person; (iii) the Indemnified Person shall have reasonably concluded that there may be legal defenses available to it that are different from or in addition to those available to the Indemnifying Person; or (iv) the named parties in any such proceeding (including any impleaded parties) include both the Indemnifying Person and the Indemnified Person and representation of both parties by the same counsel would be inappropriate due to actual or potential differing interests between them. It is understood and agreed that the Indemnifying Person shall not, in connection with any proceeding or related proceeding in the same jurisdiction, be liable for the fees and expenses of more than one separate firm (in addition to any local counsel) for all Indemnified Persons, and that all such fees and expenses shall be reimbursed as they are incurred. Any such separate firm for the Company and any control persons of the Company shall be designated in writing by the Company. The Indemnifying Person shall not be liable for any settlement of any proceeding effected without its written consent, but if settled with such consent or if there be a final judgment for the plaintiff, the Indemnifying Person agrees to indemnify each Indemnified Person from and against any loss or liability by reason of such settlement or judgment. The Indemnified Person shall notify the Indemnifying Person promptly upon any such settlement or final judgment. Notwithstanding the foregoing sentence, if at any time an Indemnified Person shall have requested that an Indemnifying Person reimburse the Indemnified Person for fees and expenses of counsel as contemplated by this paragraph, the Indemnifying Person shall be liable for any settlement of any proceeding effected without its written consent if (i) such settlement is entered into more than 30 days after receipt by the Indemnifying Person of such request and (ii) the Indemnifying Person shall not have reimbursed the Indemnified Person in accordance with such request prior to the date of such settlement. No Indemnifying Person shall, without the written consent of the Indemnified Person, effect any settlement of any pending or threatened proceeding in respect of which any Indemnified Person is or could have been a party and indemnification could have been sought hereunder by such Indemnified Person, unless such settlement (A) includes an unconditional release of such Indemnified Person, in form and substance reasonably satisfactory to such Indemnified Person, from all liability or claims that are the subject matter of such proceeding and (B) does not include any statement as to or any admission of fault, culpability or a failure to act by or on behalf of any Indemnified Person.

(d) Contribution. If the indemnification provided for in paragraphs (a) and (b) above is unavailable to an Indemnified Person or insufficient in respect of any losses, claims, damages or liabilities referred to therein, then each Indemnifying Person under such paragraph, in lieu of indemnifying such Indemnified Person thereunder, shall contribute to the amount paid or payable by such Indemnified Person as a result of such losses, claims, damages or liabilities (i) in such proportion as is appropriate to reflect the relative benefits received by the Company on the one hand and the Underwriter on the other from the offering of the Securities or (ii) if the allocation provided by clause (i) is not permitted by applicable law, in such proportion as is appropriate to reflect not only the relative benefits referred to in clause (i) but also the relative fault of the

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Company on the one hand and the Underwriter on the other in connection with the statements or omissions that resulted in such losses, claims, damages or liabilities, as well as any other relevant equitable considerations. The relative benefits received by the Company on the one hand and the Underwriter on the other shall be deemed to be in the same respective proportions as the net proceeds (before deducting expenses) received by the Company from the sale of the Securities and the total discounts and commissions received by the Underwriter in connection therewith, as provided in this Agreement, bear to the aggregate offering price of the Securities. The relative fault of the Company on the one hand and the Underwriter on the other shall be determined by reference to, among other things, whether the untrue or alleged untrue statement of a material fact or the omission or alleged omission to state a material fact relates to information supplied by the Company or by the Underwriter and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such statement or omission.

(e) Limitation on Liability. The Company and the Underwriter agree that it would not be just and equitable if contribution pursuant to this Section 8 were determined by pro rata allocation or by any other method of allocation that does not take account of the equitable considerations referred to in paragraph (d) above. The amount paid or payable by an Indemnified Person as a result of the losses, claims, damages and liabilities referred to in paragraph (d) above shall be deemed to include, subject to the limitations set forth above, any legal or other expenses incurred by such Indemnified Person in connection with any such action or claim. Notwithstanding the provisions of this Section, in no event shall the Underwriter be required to contribute any amount in excess of the amount by which the total discounts and commissions received by the Underwriter with respect to the offering of the Securities exceeds the amount of any damages that the Underwriter has otherwise been required to pay by reason of such untrue or alleged untrue statement or omission or alleged omission. No person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the Securities Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation.

(f) The remedies provided for in this Section 8 are not exclusive and shall not limit any rights or remedies that may otherwise be available to any Indemnified Person at law or in equity.

9. Termination. This Agreement may be terminated in the absolute discretion of the Underwriter, by notice to the Company, if after the execution and delivery of this Agreement and prior to the Closing Date (i) trading generally shall have been suspended or materially limited on the New York Stock Exchange or the over-the-counter market; (ii) trading of any securities issued or guaranteed by the Company shall have been suspended on any exchange or in any over-the-counter market; (iii) a general moratorium on commercial banking activities shall have been declared by federal or New York State authorities; or (iv) there shall have occurred any outbreak or escalation of hostilities or any change in financial markets or any calamity or crisis, either within or outside the United States, that, in the judgment of the Underwriter, is material and adverse and makes it impracticable or inadvisable to proceed with the offering, sale or delivery of the Securities on the terms and in the manner contemplated by this Agreement and the Prospectus.

10. Payment of Expenses. (a) Whether or not the transactions contemplated by this Agreement are consummated or this Agreement is terminated, the Company agrees to pay or

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cause to be paid all costs and expenses incident to the performance of its obligations hereunder, including without limitation, (i) the costs incident to the authorization, issuance, sale, preparation and delivery of the Securities and any taxes payable in that connection; (ii) the costs incident to the preparation and printing of the Registration Statement and the Prospectus (including any amendment or supplement thereto) and the distribution thereof; (iii) the costs of reproducing and distributing each of the Transaction Documents; (iv) the fees and expenses of the Company’s counsel and independent accountants; (v) any fees charged by rating agencies for rating the Securities; (vi) the fees and expenses of the Trustee and any paying agent (including related fees and expenses of any counsel to such parties); (vii) all expenses and application fees incurred in connection with the approval of the Securities for book-entry transfer by DTC; and (viii) all expenses incurred by the Company in connection with any “road show” presentation to potential investors. It is understood that, except as provided in Section 9(b), the Underwriter will pay all of its costs and expenses, including fees and disbursements of its counsel, in connection with this Agreement and the offering contemplated hereby.

(b) If (i) this Agreement is terminated by the Underwriter pursuant to Section 6, other than Section 6(h) hereof, or (ii) the Company for any reason, except failure to deliver the Securities as a result of the events described in Section 6(i), fails to tender the Securities for delivery to the Underwriter, the Company agrees to reimburse the Underwriter for all out-of-pocket costs and expenses (including the fees and expenses of its counsel) reasonably incurred by the Underwriter in connection with this Agreement and the offering contemplated hereby.

11. Persons Entitled to Benefit of Agreement. This Agreement shall inure to the benefit of and be binding upon the parties hereto and their respective successors and any controlling persons referred to herein, and the affiliates, officers and directors of the Underwriter and the Company. Nothing in this Agreement is intended or shall be construed to give any other person any legal or equitable right, remedy or claim under or in respect of this Agreement or any provision contained herein. No purchaser of Securities from the Underwriter shall be deemed to be a successor merely by reason of such purchase.

12. Survival. The respective indemnities, rights of contribution, representations, warranties and agreements of the Company and the Underwriter contained in this Agreement or made by or on behalf of the Company or the Underwriter pursuant to this Agreement or any certificate delivered pursuant hereto shall survive the delivery of and payment for the Securities and shall remain in full force and effect, regardless of any termination of this Agreement or any investigation made by or on behalf of the Company or the Underwriter.

13. Certain Defined Terms. For purposes of this Agreement, (a) except where otherwise expressly provided, the term “affiliate” has the meaning set forth in Rule 405 under the Securities Act; (b) the term “business day” means any day other than a day on which banks are permitted or required to be closed in New York City; and (c) the term “Exchange Act” means the Securities Exchange Act of 1934, as amended.

14. Miscellaneous. (a) Notices. All notices and other communications hereunder shall be in writing and shall be deemed to have been duly given if mailed or transmitted and confirmed by any standard form of telecommunication. Notices to the Underwriter shall be given to Morgan Stanley & Co. Incorporated, 1585 Broadway, New York, New York 10036.

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Notices to the Company shall be directed to the Company at 111 Huntington Avenue, Suite 300, Boston, Massachusetts 02199 (fax: (617)  ###-###-####); Attention: Frank D. Burt, Esq.

(b) Governing Law. This Agreement shall be governed by and construed in accordance with the laws of the State of New York.

(c) Counterparts. This Agreement may be signed in counterparts (which may include counterparts delivered by any standard form of telecommunication), each of which shall be an original and all of which together shall constitute one and the same instrument.

(d) Amendments or Waivers. No amendment or waiver of any provision of this Agreement, nor any consent or approval to any departure therefrom, shall in any event be effective unless the same shall be in writing and signed by the parties hereto.

(e) No Fiduciary Duty. The Company acknowledges that in connection with the offering of the Securities: (i) the Underwriter has acted at arms length, is not an agents of, and owes no fiduciary duties to, the Company or any other person, (ii) the Underwriter owes the Company only those duties and obligations set forth in this Agreement and prior written agreements (to the extent not superseded by this Agreement), if any, and (iii) the Underwriter may have interests that differ from those of the Company. The Company waives to the full extent permitted by applicable law any claims it may have against the Underwriter arising from an alleged breach of fiduciary duty in connection with the offering of the Securities.

(f) Headings. The headings herein are included for convenience of reference only and are not intended to be part of, or to affect the meaning or interpretation of, this Agreement.

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If the foregoing is in accordance with your understanding, please indicate your acceptance of this Agreement by signing in the space provided below.