UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

o

Rule 13d-1(b)

ý

Rule 13d-1(c)

o

Rule 13d-1(d)

CUSIP No. 812578102

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
Bill & Melinda Gates Foundation

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

o

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization
State of Washington

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
-0-

6.

Shared Voting Power 
3,521,088*

7.

Sole Dispositive Power 
-0-

8.

Shared Dispositive Power
3,521,088*

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
3,521,088*

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

11.

Percent of Class Represented by Amount in Row (9)
8.3%

12.

Type of Reporting Person (See Instructions)
OO

CUSIP No. 812578102

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
William H. Gates III

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

o

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization
United States of America

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
-0-

6.

Shared Voting Power 
3,521,088*

7.

Sole Dispositive Power 
-0-

8.

Shared Dispositive Power
3,521,088*

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
3,521,088*

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

11.

Percent of Class Represented by Amount in Row (9)
8.3%

12.

Type of Reporting Person (See Instructions)
IN

CUSIP No. 812578102

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
Melinda French Gates

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

o

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization
United States of America

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
-0-

6.

Shared Voting Power 
3,521,088*

7.

Sole Dispositive Power 
-0-

8.

Shared Dispositive Power
3,521,088*

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
3,521,088*

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

11.

Percent of Class Represented by Amount in Row (9)
8.3%

12.

Type of Reporting Person (See Instructions)
IN

Item 1.

(a)

Name of Issuer
Seattle Genetics, Inc. (the “Issuer”)

(b)

Address of Issuer’s Principal Executive Offices
21823 30th Drive SE, Bothell, WA 98021

Item 2.

(a)

Name of Person Filing
Bill & Melinda Gates Foundation (the “Foundation”), Melinda French Gates and William H. Gates III.*

(b)

Address of Principal Business Office or, if none, Residence
The Foundation – 1551 Eastlake Avenue E., Seattle, Washington  98102

Mr. Gates – One Microsoft Way, Redmond, Washington  98052

Mrs. Gates – 2365 Carillon Point, Kirkland, Washington  98033

(c)

Citizenship
The Foundation is a charitable trust organized under the laws of the State of Washington.

Both Mr. and Mrs. Gates are citizens of the United States of America.

(d)

Title of Class of Securities
Common Stock, par value $0.001 (“Common Stock”)

(e)

CUSIP Number
812578102

*Neither the present filing nor anything contained herein shall be construed as an admission that the Foundation or Mr. and Mrs. Gates constitute a “person” for any purpose other than Section 13(g) of the Securities Exchange Act of 1934, or that the Foundation and Mr. and Mrs. Gates constitute a “group” for any purpose.

Item 3.

If this statement is filed pursuant to §§240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:

(a)

o

Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o).

(b)

o

Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c).

(c)

o

Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c).

(d)

o

Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C 80a-8).

(e)

o

An investment adviser in accordance with §240.13d-1(b)(1)(ii)(E);

(f)

o

An employee benefit plan or endowment fund in accordance with §240.13d-1(b)(1)(ii)(F);

(g)

o

A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);

(h)

o

A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);

(i)

o

A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);

(j)

o

Group, in accordance with §240.13d-1(b)(1)(ii)(J).

Not Applicable.

Item 4.

Ownership

Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.

(a)

Amount beneficially owned:   

See the responses to Item 9 on the attached cover pages.  All of the shares of Common Stock reported on this Schedule 13G were acquired as a gift from Cascade Investment, L.L.C. to the Foundation on February 14, 2006

(b)

Percent of class:   

See the responses to Item 11 on the attached cover pages.

(c)

Number of shares as to which the person has:

(i)

Sole power to vote or to direct the vote   

See the responses to Item 5 on the attached cover pages.

(ii)

Shared power to vote or to direct the vote    

See the responses to Item 6 on the attached cover pages.

(iii)

Sole power to dispose or to direct the disposition of   

See the responses to Item 7 on the attached cover pages.

(iv)

Shared power to dispose or to direct the disposition of   

See the responses to Item 8 on the attached cover pages.

Item 5.

Ownership of Five Percent or Less of a Class

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following   o.

Not Applicable.

Item 6.

Ownership of More than Five Percent on Behalf of Another Person

Not Applicable.

Item 7.

Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the Parent Holding Company or Control Person

Not Applicable.

Item 8.

Identification and Classification of Members of the Group

Not Applicable.

Item 9.

Notice of Dissolution of Group

Not Applicable.

Item 10.

Certification

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

Date: February 24, 2006

BILL & MELINDA GATES FOUNDATION

By

/s/ Michael Larson

Name: Michael Larson*

Title: Attorney-in-fact for the Co-Trustees, William
H. Gates III and Melinda French Gates

WILLIAM H. GATES III

By

/s/ Michael Larson

Name: Michael Larson*

Title: Attorney-in-fact

MELINDA FRENCH GATES

By

/s/ Michael Larson

Name: Michael Larson*

Title: Attorney-in-fact

BILL & MELINDA GATES FOUNDATION

By

/s/ Michael Larson

Name: Michael Larson*

Title: Attorney-in-fact for each of the Co-Trustees, William
H. Gates III and Melinda French Gates

WILLIAM H. GATES III

By

/s/ Michael Larson

Name: Michael Larson*

Title: Attorney-in-fact

MELINDA FRENCH GATES

By

/s/ Michael Larson

Name: Michael Larson*

Title: Attorney-in-fact