POOLING AND SERVICING AGREEMENT

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EXHIBIT 4.1

EXECUTION VERSION

WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.,

as Depositor

and

WASHINGTON MUTUAL BANK
as Servicer

and

DEUTSCHE BANK NATIONAL TRUST COMPANY,

as Trustee

and

DEUTSCHE BANK TRUST COMPANY DELAWARE,

as Delaware Trustee


POOLING AND SERVICING AGREEMENT

$3,201,069,294.58

Washington Mutual Mortgage Securities Corp.

WaMu Mortgage Pass-Through Certificates

Series 2005-AR11

Cut-Off Date: August 1, 2005

This Pooling and Servicing Agreement, dated and effective as of August 1, 2005 (this “ Agreement ”), is executed by and among Washington Mutual Mortgage Securities Corp., as depositor (the “ Company ”), Washington Mutual Bank, as Servicer (the “ Servicer ”), Deutsche Bank National Trust Company, a national banking association with a corporate trust office at 1761 East St. Andrew Place, Santa Ana, CA 92705, as Trustee (the “ Trustee ”), and Deutsche Bank Trust Company Delaware, as Delaware Trustee (the “ Delaware Trustee ”).  Capitalized terms used in this Agreement and not otherwise defined have the meanings ascribed to such terms in Article I hereof.

PRELIMINARY STATEMENT

The Company at the Closing Date is the owner of the Mortgage Loans and the other property being conveyed by it to the Trust. On the Closing Date, the Company will acquire the REMIC I Regular Interests and the Class R-1 Residual Interest from the Trust as consideration for its transfer to the Trust of the Mortgage Loans and certain other assets and will be the owner of the REMIC I Regular Interests and the Class R-1 Residual Interest.  Thereafter on the Closing Date, the Company will acquire the Certificates (other than the Class R Certificates) and the Class R-2 Residual Interest from the Trust as consideration for its transfer to the Trust of the REMIC I Regular Interests and will be the owner of the Certificates.  The Company has duly authorized the execution and delivery of this Agreement to provide for (i) the conveyance to the Trust of the Mortgage Loans and certain other assets, (ii) the issuance to the Company of the REMIC I Regular Interests and the Class R-1 Residual Interest representing in the aggregate the entire beneficial interest in REMIC I, (iii) the conveyance to the Trust of the REMIC I Regular Interests and (iv) the issuance to the Company of the REMIC II Regular Interests and the Certificates, such REMIC II Regular Interests and the Class R-2 Residual Interest representing in the aggregate the entire beneficial interest in REMIC II. The Company and the Servicer are entering into this Agreement, and the Trustee and the Delaware Trustee are each accepting the trust created hereby, for good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged.

The Certificates issued hereunder, other than the Junior Subordinate Certificates and the Class PPP Certificates, have been offered for sale pursuant to a Prospectus, dated July 13, 2005, and a Prospectus Supplement, dated August 23, 2005, of the Company (together, the “ Prospectus ”). The Junior Subordinate Certificates and the Class PPP Certificates have been offered for sale pursuant to a Private Placement Memorandum, dated August 25, 2005.  The Trust created hereunder is intended to be the “Trust” described in the Prospectus and the Private Placement Memorandum and the Certificates are intended to be the “Certificates” described therein. The following tables set forth the designation, type of interest, Certificate Interest Rate, initial Class Principal Balance and Final Maturity Date for the REMIC I Regular Interests, the REMIC II Regular Interests and the Class R Residual Interests:

REMIC I Interests

As provided herein, with respect to REMIC I, the Servicer will cause an election to be made on behalf of REMIC I to be treated for federal income tax purposes as a REMIC. The REMIC I Regular Interests will be designated regular interests in REMIC I and the Class R-1 Residual Interest will be designated the sole class of residual interest in REMIC I, for purposes of the REMIC Provisions.

REMIC II Interests

As provided herein, with respect to REMIC II, the Servicer will cause an election to be made on behalf of REMIC II to be treated for federal income tax purposes as a REMIC.  The REMIC II Regular Interests will be designated regular interests in REMIC II and the Class R-2 Residual Interest will be designated the sole class of residual interest in REMIC II, for purposes of the REMIC Provisions.

In addition, the Trust will issue the Class R Certificates, which will represent ownership of the Class R-1 and Class R-2 Residual Interests.

In addition, the Trust will issue the Class A and Class B Certificates, each of which Class will represent ownership of the Corresponding Class of REMIC II Regular Interests and with respect to the each Class of Class A and Class B Certificates, will also represent ownership of the applicable rights specified in the second and fifth sentences of Section 4.04(a), and will issue the Class X Certificates, which will represent ownership of (i) the Class X-L Regular Interest and (ii) the obligations specified in the second sentence of Section 4.04(a).

In addition, the Trust will issue the Class PPP Certificates, which will not have a Class Principal Balance and will only be entitled to receive on any Distribution Date the aggregate of all Assigned Prepayment Premiums.  The Class PPP Certificates will not represent an interest in any REMIC.

As of the Cut-Off Date, the Mortgage Loans have an aggregate Principal Balance of $3,201,069,294.58 and the Certificates have an Aggregate Certificate Principal Balance of $3,201,069,294.58.

W I T N E S S E T H :

WHEREAS, the Company is a corporation duly organized and existing under and by virtue of the laws of the State of Delaware and has full corporate power and authority to enter into this Agreement and to undertake the obligations undertaken by it herein;

WHEREAS, the Servicer is a federal savings association duly organized and existing under and by virtue of the laws of the United States of America and has full power and authority to enter into this Agreement and to undertake the obligations undertaken by it herein;

WHEREAS, the Trustee is a national banking association duly organized and existing under the laws of the United States of America and has full power and authority to enter into this Agreement;

WHEREAS, the Delaware Trustee is a banking corporation duly organized and existing under the laws of the State of Delaware and has full power and authority to enter into this Agreement;

WHEREAS, prior to the execution and delivery hereof, the Company and the Delaware Trustee have entered into the Original Trust Agreement, and the Delaware Trustee has filed the Certificate of Trust;

WHEREAS, it is the intention of the Company, the Servicer, the Trustee and the Delaware Trustee that the Trust created by this Agreement constitute a statutory trust under the Statutory Trust Statute, that this Agreement constitute the governing instrument of the Trust, and that this Agreement amend and restate the Original Trust Agreement;

WHEREAS, the Company is the owner of the Mortgage Loans identified in the Mortgage Loan Schedule hereto having unpaid Principal Balances on the Cut-Off Date as stated therein; and

WHEREAS, the Company has been duly authorized to create the Trust to (i) hold the Mortgage Loans and certain other property, (ii) issue the REMIC I Regular Interests and the Class R-1 Residual Interest, (iii) hold the REMIC I Regular Interests and (iv) issue the REMIC II Regular Interests and the Certificates.

NOW, THEREFORE, in order to declare the terms and conditions upon which the REMIC I Regular Interests, the REMIC II Regular Interests, the Class R Residual Interests and the Certificates are to be issued, and in consideration of the premises and of the purchase and acceptance of the Certificates by the Holders thereof, the Company covenants and agrees with the Servicer, the Trustee and the Delaware Trustee, for the equal and proportionate benefit of the respective Holders from time to time of the REMIC I Regular Interests, the REMIC II Regular Interests and the Certificates, as applicable, as follows:

ARTICLE I

Section 1.01.        Definitions .

Whenever used in this Agreement, the following words and phrases, unless the context otherwise requires, shall have the following meanings:

Adjusted Cap Rate :  For any Distribution Date and any Class of Class A-L Regular Interests, a fraction, expressed as a per annum rate, the numerator of which is equal to the product of (i) the amount of interest distributions accrued on the Mortgage Loans at the Weighted Average Pass-Through Rate for that Distribution Date less the Net Negative Amortization Amount and (ii) 12, and the denominator of which is equal to the aggregate Principal Balance of the Mortgage Loans as of the second preceding Due Date after giving effect to the payments due on the Mortgage Loans on that Due Date multiplied, such fraction multiplied by a ratio, the numerator of which is 30 and the denominator of which is the actual number of days in the related certificate accrual period.

For any Distribution Date and any Class of Class B-L Regular Interests, the Weighted Average Pass-Through Rate, computed for this purpose by (i) reducing the Weighted Average Pass-Through Rate by a per annum rate equal to a fraction, the numerator of which is the Net Negative Amortization Amount with respect to the Mortgage Loans multiplied by 12, and the denominator of which is the aggregate Principal Balance of the Mortgage Loans as of the second preceding Due Date after giving effect to the payments due on the Mortgage Loans on that Due Date.

Adjusted Weighted Average Pass-Through Rate :  For any Distribution Date, the product of (i) the Weighted Average Pass-Through Rate for such Distribution Date and (ii) a fraction, the numerator of which is 30 and the denominator of which is the actual number of days in the related No-Delay Accrual Period.

Aggregate Certificate Principal Balance :  At any given time, the sum of the then current Class Principal Balances of the Certificates.

Appraised Value :  With respect to any (i) Mortgage Loan that is not a Streamlined Mortgage Loan or ROV Mortgage Loan, the lesser of (a) the value set forth on the appraisal made in connection with the origination of such Mortgage Loan as the value of the related Mortgaged Property and (b) the purchase price paid for the Mortgaged Property, provided , however , that if such Mortgage Loan was originated in connection with the refinance of a mortgage loan, such value shall be based solely on the appraisal made in connection with the origination of such Mortgage Loan; (ii) ROV Mortgage Loan, the lesser of (a) the value set forth on the residential appraisal review made in connection with the origination of such Mortgage Loan as the value of the related Mortgaged Property and (b) the purchase price paid for the Mortgaged Property, provided , however , that if such ROV Mortgage Loan was originated in connection with the refinance of a mortgage loan, such value shall be based solely on the residential appraisal review made in connection with the origination of such ROV Mortgage Loan; and (iii) Streamlined Mortgage Loan, the value set forth in the appraisal made in connection with the origination of the mortgage loan being refinanced.

Assigned Prepayment Premiums :  For any Distribution Date, any Prepayment Premium on a Mortgage Loan and any other Prepayment Premium on deposit in the Certificate Account received in the Prior Period.

Assignment of Proprietary Lease :  With respect to a Cooperative Loan, the assignment or mortgage of the related Cooperative Lease from the Mortgagor to the originator of the Cooperative Loan.

Authenticating Agent :  Any authenticating agent appointed by the Trustee pursuant to Section 8.11.

Authorized Denomination :  With respect to each Class of Certificates (other than the Class X and Class R Certificates), an initial Certificate Principal Balance equal to $25,000 and multiples of $1 in excess thereof, except that one Certificate of each Class of the Junior Subordinate Certificates may be issued in an amount that is not an integral multiple of $1.  With respect to the Class X Certificates, a Class Notional Amount as of the Cut-Off Date equal to $100,000 and multiples of $1 in excess thereof.  With respect to the Class R Certificates, one Certificate with a Percentage Interest equal to 0.01% and one Certificate with a Percentage Interest equal to 99.99%.  The Class PPP Certificates will be issued in fully registered form in minimum denominations of 20% of the Percentage Interest therein and increments of 10% in excess thereof.

Bankruptcy Loss :  A loss on a Mortgage Loan arising out of (i) a reduction in the Minimum Monthly Payment for such Mortgage Loan by a court of competent jurisdiction in a case under the United States Bankruptcy Code, other than any such reduction that arises out of clause (ii) of this definition of “Bankruptcy Loss,” including, without limitation, any such reduction that results in a permanent forgiveness of principal, or (ii) with respect to any Mortgage Loan, a valuation, by a court of competent jurisdiction in a case under such Bankruptcy Code, of the related Mortgaged Property in an amount less than the then outstanding Principal Balance of such Mortgage Loan.

Beneficial Holder :  A Person holding a beneficial interest in any Book-Entry Certificate as or through a DTC Participant or an Indirect DTC Participant or a Person holding a beneficial interest in any Definitive Certificate.

Benefit Plan Opinion :  With respect to any Certificate presented for registration in the name of any Person, an Opinion of Counsel acceptable to and in form and substance satisfactory to the Trustee and the Company to the effect that the purchase or holding of such Certificate is permissible under applicable law, will not constitute or result in a non-exempt prohibited transaction under Section 406 of ERISA or Section 4975 of the Code, and will not subject the Trust, the Trustee, the Delaware Trustee, the Servicer or the Company to any obligation or liability (including obligations or liabilities under Section 406 of ERISA or Section 4975 of the Code) in addition to those undertaken in this Agreement, which Opinion of Counsel shall not be an expense of the Trust, the Trustee, the Delaware Trustee, the Servicer or the Company.

Book-Entry Certificates :  The Class A, Class X and Senior Subordinate Certificates, beneficial ownership and transfers of which shall be made through book entries as described in Section 5.07.

BSFP :  Bear Stearns Financial Products Inc.

Business Day :  Any day other than a Saturday or a Sunday or a day on which banking institutions in Stockton, California, Chicago, Illinois, New York, New York, Seattle, Washington or any city in which the Corporate Trust Office is located (which shall initially be Santa Ana, California) are authorized or obligated by law or executive order to be closed.

Buydown Agreement :  An agreement between a Person and a Mortgagor pursuant to which such Person has provided a Buydown Fund.

Buydown Fund :  A fund provided by the originator of a Mortgage Loan or another Person with respect to a Buydown Loan which provides an amount sufficient to subsidize regularly scheduled principal and interest payments due on such Buydown Loan for a period.  Buydown Funds may be (i) funded at the par values of future payment subsidies, or (ii) funded in an amount less than the par values of future payment subsidies, and determined by discounting such par values in accordance with interest accruing on such amounts, in which event they will be deposited in an account bearing interest. Buydown Funds may be held in a separate Buydown Fund Account or may be held in a Custodial Account for P&I or a Custodial Account for Reserves and monitored by the Servicer.

Buydown Fund Account :  A separate account or accounts created and maintained pursuant to Section 3.02 (a) with a financial institution approved by the Servicer, (b) within FDIC insured accounts (or other accounts with comparable insurance coverage acceptable to the Rating Agencies) created, maintained and monitored by the Servicer or (c) in a separate non-trust account without FDIC or other insurance in an Eligible Institution (including the Trustee). Such account or accounts may be non-interest bearing or may bear interest. In the event that a Buydown Fund Account is established pursuant to clause (b) of the preceding sentence, amounts held in such Buydown Fund Account shall not exceed the level of deposit insurance coverage on such account; accordingly, more than one Buydown Fund Account may be established.

Buydown Loan :  A Mortgage Loan for which the Mortgage Interest Rate has been subsidized through a Buydown Fund provided at the time of origination of such Mortgage Loan.

Cap Counterparty :  BSFP.

Cap Strike Rate :  For any Distribution Date, the amount set forth under the heading “Strike Rate” in Schedule 1 of the Prospectus.

Carry-Forward Subsequent Recoveries Amount :  For any Distribution Date, the excess, if any, of (i) the Subsequent Recoveries for such Distribution Date over (ii) the amount by which the Class Principal Balance of the Class of Subordinate Certificates with the lowest priority is increased in respect of Subsequent Recoveries on such Distribution Date pursuant to the definition of “Class Principal Balance” herein.

Carryover Shortfall Amount :  For any Distribution Date and for any Class of Class A Certificates and any Class of Class B Certificates, the sum of: (i) the excess, if any, of (a) the amount of interest that would have accrued on the Class Principal Balance of such Class’ Corresponding Class immediately before such Distribution Date, during the No-Delay Accrual Period, at a Certificate Interest Rate equal to the lesser of (1) LIBOR plus the related margin for such Class for such Distribution Date (as specified in the applicable note to the table entitled “REMIC II Interests” in the Preliminary Statement hereto) and (2) 10.50%, over (b) the amount of interest that accrued on such Class Principal Balance, during the No-Delay Accrual Period, at the actual Certificate Interest Rate for such Class for such Distribution Date, (ii) the portion of the amount described in clause (i) above remaining unpaid from prior Distribution Dates, and (iii) one month’s interest at the Certificate Interest Rate described in clause (i)(a) above on the amount described in clause (ii) above.

Carryover Shortfall Payment :  For any Class of Class A Certificates for any Distribution Date, the lesser of (a) the Carryover Shortfall Amount for such Class for such Distribution Date reduced by the Yield Maintenance Payment for such Class for such Distribution Date and (b) such Class’ pro rata share of the Interest Distribution Amount for the Class X-L Regular Interest for such Distribution Date (such pro rata share calculated based on an allocation of such Interest Distribution Amount among the Classes of Class A Certificates pro rata according to Carryover Shortfall Amount for such Distribution Date).

For the Class B-1 Certificates for any Distribution Date, the lesser of (a) the Carryover Shortfall Amount for such Class for such Distribution Date and (b) the excess, if any, of (i) the Interest Distribution Amount for the Class X-L Regular Interest for such Distribution Date over (ii) the aggregate of the Carryover Shortfall Payments for the Class A Certificates for such Distribution Date.

For the Class B-2 Certificates for any Distribution Date, the lesser of (a) the Carryover Shortfall Amount for such Class for such Distribution Date and (b) the excess, if any, of (i) the Interest Distribution Amount for the Class X-L Regular Interest for such Distribution Date over (ii) the aggregate of the Carryover Shortfall Payments for the Class A and Class B-1 Certificates for such Distribution Date.

For the Class B-3 Certificates for any Distribution Date, the lesser of (a) the Carryover Shortfall Amount for such Class for such Distribution Date and (b) the excess, if any, of (i) the Interest Distribution Amount for the Class X-L Regular Interest for such Distribution Date over (ii) the aggregate of the Carryover Shortfall Payments for the Class A, Class B-1 and Class B-2 Certificates for such Distribution Date.

For the Class B-4 Certificates for any Distribution Date, the lesser of (a) the Carryover Shortfall Amount for such Class for such Distribution Date and (b) the excess, if any, of (i) the Interest Distribution Amount for the Class X-L Regular Interest for such Distribution Date over (ii) the aggregate of the Carryover Shortfall Payments for the Class A, Class B-1, Class B-2 and Class B-3 Certificates for such Distribution Date.

For the Class B-5 Certificates for any Distribution Date, the lesser of (a) the Carryover Shortfall Amount for such Class for such Distribution Date and (b) the excess, if any, of (i) the Interest Distribution Amount for the Class X-L Regular Interest for such Distribution Date over (ii) the aggregate of the Carryover Shortfall Payments for the Class A, Class B-1, Class B-2, Class B-3 and Class B-4 Certificates for such Distribution Date.

For the Class B-6 Certificates for any Distribution Date, the lesser of (a) the Carryover Shortfall Amount for such Class for such Distribution Date and (b) the excess, if any, of (i) the Interest Distribution Amount for the Class X-L Regular Interest for such Distribution Date over (ii) the aggregate of the Carryover Shortfall Payments for the Class A, Class B-1, Class B-2, Class B-3, Class B-4 and Class B-5 Certificates for such Distribution Date.

For the Class B-7 Certificates for any Distribution Date, the lesser of (a) the Carryover Shortfall Amount for such Class for such Distribution Date and (b) the excess, if any, of (i) the Interest Distribution Amount for the Class X-L Regular Interest for such Distribution Date over (ii) the aggregate of the Carryover Shortfall Payments for the Class A, Class B-1, Class B-2, Class B-3, Class B-4, Class B-5 and Class B-6 Certificates for such Distribution Date.

For the Class B-8 Certificates for any Distribution Date, the lesser of (a) the Carryover Shortfall Amount for such Class for such Distribution Date and (b) the excess, if any, of (i) the Interest Distribution Amount for the Class X-L Regular Interest for such Distribution Date over (ii) the aggregate of the Carryover Shortfall Payments for the Class A, Class B-1, Class B-2, Class B-3, Class B-4, Class B-5, Class B-6 and Class B-7 Certificates for such Distribution Date.

For the Class B-9 Certificates for any Distribution Date, the lesser of (a) the Carryover Shortfall Amount for such Class for such Distribution Date and (b) the excess, if any, of (i) the Interest Distribution Amount for the Class X-L Regular Interest for such Distribution Date over (ii) the aggregate of the Carryover Shortfall Payments for the Class A, Class B-1, Class B-2, Class B-3, Class B-4, Class B-5, Class B-6, Class B-7 and Class B-8 Certificates for such Distribution Date.

For the Class B-10 Certificates for any Distribution Date, the lesser of (a) the Carryover Shortfall Amount for such Class for such Distribution Date and (b) the excess, if any, of (i) the Interest Distribution Amount for the Class X-L Regular Interest for such Distribution Date over (ii) the aggregate of the Carryover Shortfall Payments for the Class A, Class B-1, Class B-2, Class B-3, Class B-4, Class B-5, Class B-6, Class B-7, Class B-8 and Class B-9 Certificates for such Distribution Date.

For the Class B-11 Certificates for any Distribution Date, the lesser of (a) the Carryover Shortfall Amount for such Class for such Distribution Date and (b) the excess, if any, of (i) the Interest Distribution Amount for the Class X-L Regular Interest for such Distribution Date over (ii) the aggregate of the Carryover Shortfall Payments for the Class A, Class B-1, Class B-2, Class B-3, Class B-4, Class B-5, Class B-6, Class B-7, Class B-8, Class B-9 and Class B-10 Certificates for such Distribution Date.

For the Class B-12 Certificates for any Distribution Date, the lesser of (a) the Carryover Shortfall Amount for such Class for such Distribution Date and (b) the excess, if any, of (i) the Interest Distribution Amount for the Class X-L Regular Interest for such Distribution Date over (ii) the aggregate of the Carryover Shortfall Payments for the Class A, Class B-1, Class B-2, Class B-3, Class B-4, Class B-5, Class B-6, Class B-7, Class B-8, Class B-9, Class B-10 and Class B-11 Certificates for such Distribution Date.

For the Class B-13 Certificates for any Distribution Date, the lesser of (a) the Carryover Shortfall Amount for such Class for such Distribution Date and (b) the excess, if any, of (i) the Interest Distribution Amount for the Class X-L Regular Interest for such Distribution Date over (ii) the aggregate of the Carryover Shortfall Payments for the Class A, Class B-1, Class B-2, Class B-3, Class B-4, Class B-5, Class B-6, Class B-7, Class B-8, Class B-9, Class B-10, Class B-11 and Class B-12 Certificates for such Distribution Date.

Certificate :  Any one of the Certificates issued pursuant to this Agreement, executed by the Trustee and authenticated by or on behalf of the Trustee hereunder in substantially one of the forms set forth in Exhibit A and B hereto. The additional matter appearing in Exhibit H shall be deemed incorporated into Exhibit A as though set forth at the end of such Exhibit.

Certificate Account :  The separate trust account created and maintained with the Trustee, the Investment Depository or any other bank or trust company acceptable to the Rating Agencies which is incorporated under the laws of the United States of America or any state thereof pursuant to Section 3.04, which account shall bear a designation clearly indicating that the funds deposited therein are held in trust for the benefit of the Trust or any other account serving a similar function acceptable to the Rating Agencies. Funds in the Certificate Account may be invested in Eligible Investments pursuant to Section 3.04(b) and reinvestment earnings thereon shall be paid to the Servicer as additional servicing compensation. Funds deposited in the Certificate Account (exclusive of the Servicing Fee) shall be held in trust for the Certificateholders and for the uses and purposes set forth in Section 2.01, Section 3.04, Section 3.05, Section 4.01 and Section 4.04.

Certificateholder or Holder :  With respect to the Certificates, the Person in whose name a Certificate is registered in the Certificate Register, except that, solely for the purposes of giving any consent pursuant to this Agreement, any Certificate registered in the name of the Company, the Servicer or any affiliate thereof shall be deemed not to be outstanding and the Percentage Interest evidenced thereby shall not be taken into account in determining whether the requisite percentage of Percentage Interests necessary to effect any such consent has been obtained; provided, that the Trustee may conclusively rely upon an Officer’s Certificate to determine whether any Person is an affiliate of the Company or the Servicer.  With respect to the REMIC I Regular Interests, the owner of the REMIC I Regular Interests, which as of the Closing Date shall be the Trust.  With respect to each Class of REMIC II Regular Interests, the Holder of the Corresponding Class of Certificates.

Certificate Interest Rate :  For each Class of REMIC I Regular Interests and REMIC II Regular Interests and the Class R-1 Residual Interest, the per annum rate set forth as the Certificate Interest Rate for such Class in the Preliminary Statement hereto.

Certificate of Trust :  The certificate of trust filed with respect to the Trust with the Secretary of State in accordance with Section 3810(a) of the Statutory Trust Statute.

Certificate Principal Balance :  For each Certificate of any Class, the portion of the related Class Principal Balance, if any, represented by such Certificate; provided , however , that each Class X Certificate will represent a portion of the Class X PO Principal Balance equal to its Percentage Interest in the Class X-L Notional Amount.

Certificate Register and Certificate Registrar :  The register maintained and the registrar appointed, respectively, pursuant to Section 5.03.

Class :  All REMIC I Regular Interests or the Class R-1 Residual Interest having the same priority and rights to payments on the Mortgage Loans from the REMIC I Available Distribution Amount, and all REMIC II Regular Interests or the Class R-2 Residual Interest having the same priority and rights to payments on the REMIC I Regular Interests from the REMIC II Available Distribution Amount, as applicable, which REMIC I Regular Interests, REMIC II Regular Interests and Class R Residual Interests, as applicable, shall be designated as a separate Class, and which, in the case of the Certificates (including the Class R Certificates representing ownership of the Class R Residual Interests), shall be set forth in the applicable forms of Certificates attached hereto as Exhibits A and B.  Each Class of REMIC I Regular Interests and the Class R-1 Residual Interest shall be entitled to receive the amounts allocated to such Class pursuant to the definition of “REMIC I Distribution Amount” only to the extent of the REMIC I Available Distribution Amount for such Distribution Date remaining after distributions in accordance with prior clauses of the definition of “REMIC I Distribution Amount” and each Class of REMIC II Regular Interests and the Class R-2 Residual Interest shall be entitled to receive the amounts allocated to such Class pursuant to the definition of “REMIC II Distribution Amount” only to the extent of the REMIC II Available Distribution Amount for such Distribution Date remaining after distributions in accordance with prior clauses of the definition of “REMIC II Distribution Amount.” The Class PPP Certificates shall only be entitled to receive on any Distribution Date the aggregate of all Assigned Prepayment Premiums as specified in Section 4.04(a).

In addition to their rights to receive payments from the REMIC II Available Distribution Amount on their Corresponding Class of REMIC II Regular Interests, (i) the Class A Certificates shall be entitled to receive payments, if any, as specified in the second and fifth sentences of Section 4.04(a) and (ii) the Class B Certificates shall be entitled to receive payments, if any, as specified in the second sentence of Section 4.04(a).  Notwithstanding the right of the Class X Certificates to receive payments from the REMIC II Available Distribution Amount on the Class X-L Regular Interest, the amount of such payments may be reduced as specified in the second sentence of Section 4.04(a).

Class A Certificates :  The Class A-1A, Class A-1B1, Class A-1B2, Class A-1B3, Class A-1C1, Class A-1C2, Class A-1C3 and Class A-1C4 Certificates.

Class A-L Regular Interests :  The Class A-1A-L, Class A-1B1-L, Class A-1B2-L, Class A-1B3-L, Class A-1C1-L, Class A-1C2-L, Class A-1C3-L and Class A-1C4-L Regular Interests.

Class A-1A Certificates :  The Certificates. designated as “Class A-1A” on the face thereof in substantially the form attached hereto as Exhibit A.

Class A-1A-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class A-1B1 Certificates :  The Certificates. designated as “Class A-1B1” on the face thereof in substantially the form attached hereto as Exhibit A.

Class A-1B1-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class A-1B2 Certificates :  The Certificates. designated as “Class A-1B2” on the face thereof in substantially the form attached hereto as Exhibit A.

Class A-1B2-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class A-1B3 Certificates :  The Certificates. designated as “Class A-1B3” on the face thereof in substantially the form attached hereto as Exhibit A.

Class A-1B3-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class A-1C1 Certificates :  The Certificates. designated as “Class A-1C1” on the face thereof in substantially the form attached hereto as Exhibit A.

Class A-1C1-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class A-1C2 Certificates :  The Certificates. designated as “Class A-1C2” on the face thereof in substantially the form attached hereto as Exhibit A.

Class A-1C2-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class A-1C3 Certificates :  The Certificates. designated as “Class A-1C3” on the face thereof in substantially the form attached hereto as Exhibit A.

Class A-1C3-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class A-1C4 Certificates :  The Certificates. designated as “Class A-1C4” on the face thereof in substantially the form attached hereto as Exhibit A.

Class A-1C4-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class B Certificates : The Class B-1, Class B-2, Class B-3, Class B-4, Class B-5, Class B-6, Class B-7, Class B-8, Class B-9, Class B-10, Class B-11, Class B-12 and Class B-13 Certificates.

Class B-1 Certificates :  The Certificates designated as “Class B-1” on the face thereof in substantially the form attached hereto as Exhibit A.

Class B-L Regular Interests :  The Class B-1-L, Class B-2-L, Class B-3-L, Class B-4-L, Class B-5-L, Class B-6-L, Class B-7-L, Class B-8-L, Class B-9-L, Class B-10-L, Class B-11-L, Class B-12-L and Class B-13-L Regular Interests.

Class B-1-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class B-2 Certificates :  The Certificates designated as “Class B-2” on the face thereof in substantially the form attached hereto as Exhibit A.

Class B-2-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class B-3 Certificates :  The Certificates designated as “Class B-3” on the face thereof in substantially the form attached hereto as Exhibit A.

Class B-3-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class B-4 Certificates :  The Certificates designated as “Class B-4” on the face thereof in substantially the form attached hereto as Exhibit A.

Class B-4-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class B-5 Certificates :  The Certificates designated as “Class B-5” on the face thereof in substantially the form attached hereto as Exhibit A.

Class B-5-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class B-6 Certificates :  The Certificates designated as “Class B-6” on the face thereof in substantially the form attached hereto as Exhibit A.

Class B-6-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class B-7 Certificates :  The Certificates designated as “Class B-7” on the face thereof in substantially the form attached hereto as Exhibit A.

Class B-7-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class B-8 Certificates :  The Certificates designated as “Class B-8” on the face thereof in substantially the form attached hereto as Exhibit A.

Class B-8-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class B-9 Certificates :  The Certificates designated as “Class B-9” on the face thereof in substantially the form attached hereto as Exhibit A.

Class B-9-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class B-10 Certificates :  The Certificates designated as “Class B-10” on the face thereof in substantially the form attached hereto as Exhibit A.

Class B-10-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class B-11 Certificates :  The Certificates designated as “Class B-11” on the face thereof in substantially the form attached hereto as Exhibit A.

Class B-11-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class B-12 Certificates :  The Certificates designated as “Class B-12” on the face thereof in substantially the form attached hereto as Exhibit A.

Class B-12-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class B-13 Certificates :  The Certificates designated as “Class B-13” on the face thereof in substantially the form attached hereto as Exhibit A.

Class B-13-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class LT Principal Reduction Amounts:  For any Distribution Date, the amounts by which the Class Principal Balances of the Class LT1, Class LT2, Class LT3 and Class LT4 Regular Interests, respectively, will be reduced on such Distribution Date by the allocation of Realized Losses and the distribution of principal, determined as described in Appendix 1.

Class LT Regular Interests : The Class LT1, Class LT2, Class LT3 and Class LT4 Regular Interests.

Class LT1 Regular Interest : The uncertificated undivided beneficial interest in REMIC I which constitutes a REMIC I Regular Interest and is entitled to distributions as set forth herein.

Class LT2 Principal Distribution Amount:  For any Distribution Date, the excess, if any, of the Class LT2 Principal Reduction Amount for such Distribution Date over the principal portion of Realized Losses allocated to the Class LT2 Regular Interest on such Distribution Date.

Class LT2 Regular Interest : The uncertificated undivided beneficial interest in REMIC I which constitutes a REMIC I Regular Interest and is entitled to distributions as set forth herein.

Class LT3 Principal Distribution Amount:  For any Distribution Date, the excess, if any, of the Class LT3 Principal Reduction Amount for such Distribution Date over the principal portion of Realized Losses allocated to the Class LT3 Regular Interest on such Distribution Date.

Class LT3 Regular Interest : The uncertificated undivided beneficial interest in REMIC I which constitutes a REMIC I Regular Interest and is entitled to distributions as set forth herein.

Class LT4 Principal Distribution Amount:  For any Distribution Date, the excess, if any, of the Class LT4 Principal Reduction Amount for such Distribution Date over the principal portion of Realized Losses allocated to the Class LT4 Regular Interest on such Distribution Date.

Class LT4 Regular Interest : The uncertificated undivided beneficial interest in REMIC I which constitutes a REMIC I Regular Interest and is entitled to distributions as set forth herein.

Class Notional Amount :  With respect to the Class X Certificates and the Class X-L Regular Interest, the Class X-L Notional Amount.

Class PPP Certificates :  The Certificates designated as “Class PPP” on the face thereof in substantially the form attached hereto as Exhibit A.

Class Principal Balance :  For any Class of REMIC I or REMIC II Regular Interests and for the Class R-1 Residual Interest, the applicable initial Class Principal Balance therefor set forth in the Preliminary Statement hereto (or, in the case of the Class R Certificates, the Class Principal Balance of the Class R-1 Residual Interest), corresponding to the rights of such Class in payments of principal due to be passed through to such Class from principal payments on the Mortgage Loans or the REMIC I Regular Interests, as applicable, as reduced from time to time by (x) distributions of principal to such Class and (y) the portion of Realized Losses allocated to the Class Principal Balance of such Class pursuant to the definition of “Realized Loss” with respect to a given Distribution Date, and as increased from time to time by the portion of Net Negative Amortization Amounts allocated to the Class Principal Balance of such Class pursuant to the definition of “Net Negative Amortization Amount” with respect to a given Distribution Date; and for any Class of Certificates or Components, as applicable, the Class Principal Balance of the Corresponding Class of REMIC II Regular Interests.  For any Distribution Date, the reduction of the Class Principal Balance of any Class of Certificates or Components, as applicable, and REMIC I or REMIC II Regular Interests pursuant to the definition of “Realized Loss” and the increase in the Class Principal Balance of any Class of Certificates or Components, as applicable, and REMIC I or REMIC II Regular Interests pursuant to the definition of “Net Negative Amortization Amount” shall be deemed effective after the determination and distribution of principal on such Class pursuant to the definitions of “REMIC I Distribution Amount” and “REMIC II Distribution Amount.”

Notwithstanding the foregoing, (A) any amounts distributed in respect of principal losses pursuant to paragraph (I) (xxxxiii) of the definition of “REMIC II Distribution Amount” shall not cause a reduction in the Class Principal Balances of the REMIC II Regular Interests or their Corresponding Classes and (B) any amounts distributed in respect of principal losses pursuant to clause (v) of the definition of “REMIC I Distribution Amount” shall not cause a reduction in the Class Principal Balances of the REMIC I Regular Interests. 

In addition to the foregoing, on each Distribution Date, the Class Principal Balance of the Class of Subordinate Certificates with the lowest priority then outstanding (and its Corresponding Class of REMIC II Regular Interests) shall be increased by an amount equal to the lesser of (i) the Subsequent Recoveries for such Distribution Date and (ii) the amount of Realized Losses allocated to such Class on previous Distribution Dates (the amount in this clause (ii) reduced by the amount, if any, by which such Class Principal Balance has been increased on prior Distribution Dates pursuant to this paragraph).

The Class Principal Balance for the Class A-1A Certificates shall be referred to as the “Class A-1A Principal Balance,” the Class Principal Balance for the Class A-1A-L Regular Interest shall be referred to as the “Class A-1A-L Principal Balance” and so on.  The Class Principal Balances for the Class X PO Component of the Class X-L Regular Interest shall be zero as of the Closing Date and shall increase after the Closing Date by the portion, if any, of Net Negative Amortization Amounts allocated to the Class X-L Regular Interest pursuant to the definition of “Net Negative Amortization Amount”.

Class R Certificates :  The Certificates designated as “Class R” on the face thereof in substantially the form attached hereto as Exhibit B, representing ownership of the Class R-1 and Class R-2 Residual Interests, each of which Class of Residual Interests has been designated as the sole class of “residual interest” in REMIC I and REMIC II, respectively, pursuant to Section 2.06 and Section 2.11, respectively, for purposes of Section 860G(a)(2) of the Code.

Class R Residual Interests :  The Class R-1 and Class R-2 Residual Interests (which shall be transferable only as a unit evidenced by the Class R Certificates, in accordance with the applicable provisions of Section 5.01).

Class R-1 Residual Interest :  The uncertificated undivided beneficial interest in REMIC I which has been designated as the single class of “residual interest” in REMIC I pursuant to Section 2.06.  The Class R-1 Residual Interest, together with the REMIC I Regular Interests, shall be deemed to be a separate series of beneficial interests in the assets of the Trust consisting of the REMIC I Assets pursuant to Section 3806(b)(2) of the Statutory Trust Statute.

Class R-2 Residual Interest :  The uncertificated undivided beneficial interest in REMIC II which has been designated as the single class of “residual interest” in REMIC II pursuant to Section 2.11.  The Class R-2 Residual Interest, together with the REMIC II Regular Interests, shall be deemed to be a separate series of beneficial interests in the assets of the Trust consisting of the REMIC II Assets pursuant to Section 3806(b)(2) of the Statutory Trust Statute.

Class X Certificates :  The Certificates designated as “Class X” on the face thereof in substantially the form attached hereto as Exhibit A.

Class X-L Notional Amount :  For any Distribution Date, the aggregate principal balance of the Mortgage Loans as of the second preceding Due Date after giving effect to the payments due on the Mortgage Loans on that Due Date.

Class X-L Regular Interest :  The uncertificated undivided beneficial interest in REMIC II which constitutes a REMIC II Regular Interest and is entitled to distributions as set forth herein.

Class X PO Component :  The principal-only component of the Class X Certificate; in the event interest accrued on the Class X-L Notional Amount is reduced as a result of the Net Negative Amortization Amount, such amount will be added as principal to the outstanding component principal balance of the Class X PO Component. The principal balance of the Class X PO Component will initially be zero.

Clean-Up Call Option Date :  The date on which the aggregate principal balance of the Mortgage Loans has been reduced to less than the Clean-Up Call Percentage of that balance as of the Cut-Off Date.

Clean-Up Call Percentage :  10%.

Clearing Agency :  An organization registered as a “clearing agency” pursuant to Section 17A of the Securities Exchange Act of 1934, as amended, which initially shall be DTC.

Closing Date :  August 25, 2005, which is the date of settlement of the sale of the Certificates to the original purchasers thereof.

Closing Date Loan-to-Value Ratio : For each Mortgage Loan, the principal balance of such Mortgage Loan as of the Cut-Off Date (after application of all scheduled principal payments due on or before the Cut-Off Date) divided by the value of the related Mortgaged Property as of the Closing Date.

Code :  The Internal Revenue Code of 1986, as amended.

Company :  Washington Mutual Mortgage Securities Corp., a Delaware corporation, or its successor-in-interest.

Compensating Interest : For any Distribution Date, the least of (i) the sum of (a) 1/12 of 0.050% of the aggregate Principal Balance of the Mortgage Loans immediately before such Distribution Date, (b) the aggregate Payoff Earnings with respect to the Mortgage Loans for such Distribution Date and (c) the aggregate Payoff Interest with respect to the Mortgage Loans for such Distribution Date; (ii) the aggregate Uncollected Interest with respect to the Mortgage Loans for such Distribution Date and (iii) 1/12 of 0.125% of the aggregate Principal Balance of the Mortgage Loans immediately before such Distribution Date.

Cooperative :  A private, cooperative housing corporation which owns or leases land and all or part of a building or buildings, including apartments, spaces used for commercial purposes and common areas therein and whose board of directors authorizes, among other things, the sale of Cooperative Stock.

Cooperative Apartment :  A dwelling unit in a multi-dwelling building owned or leased by a Cooperative, which unit the Mortgagor has an exclusive right to occupy pursuant to the terms of a proprietary lease or occupancy agreement.

Cooperative Lease :  With respect to a Cooperative Loan, the proprietary lease or occupancy agreement with respect to the Cooperative Apartment occupied by the Mortgagor and relating to the related Cooperative Stock, which lease or agreement confers an exclusive right to the holder of such Cooperative Stock to occupy such apartment.

Cooperative Loans :  Any of the Mortgage Loans made in respect of a Cooperative Apartment, evidenced by a Mortgage Note and secured by (i) a Security Agreement, (ii) the related Cooperative Stock Certificate, (iii) an assignment or mortgage of the Cooperative Lease, (iv) financing statements and (v) a stock power (or other similar instrument), and ancillary thereto, a Recognition Agreement, each of which was transferred and assigned to the Trust pursuant to Section 2.04.

Cooperative Stock :  With respect to a Cooperative Loan, the single outstanding class of stock, partnership interest or other ownership instrument in the related Cooperative.

Cooperative Stock Certificate :  With respect to a Cooperative Loan, the stock certificate or other instrument evidencing the related Cooperative Stock.

Corporate Trust Office :  The corporate trust office of the Trustee in the State of California, at which at any particular time its corporate trust business with respect to this Agreement shall be administered, which office at the date of the execution of this Agreement is located at 1761 East St. Andrew Place, Santa Ana, CA 92705, Attention: Trust Administration WA05AA.

Corporation :  Any Person (other than an individual, partnership, joint venture or unincorporated organization) incorporated, associated, organized, chartered or existing under the laws of any state or under the federal laws of the United States of America; provided , that such Person have indefinite existence under the law of its domicile.

Corresponding Class :  With respect to the Class A, Class X and Class B Certificates and the REMIC II Regular Interests, the “Corresponding Class” shall be as indicated in the following table: