UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

x

Rule 13d-1(b)

o

Rule 13d-1(c)

o

Rule 13d-1(d)

CUSIP No. 015351109

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
Sectoral Asset Management Inc.

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
Canada

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
203,302

6.

Shared Voting Power
2,060,432

7.

Sole Dispositive Power
2,667,134

8.

Shared Dispositive Power
-0-

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
2,667,134

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)
7.7%

12.

Type of Reporting Person (See Instructions)
IA

CUSIP No. 015351109

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
Jérôme G. Pfund

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
Swiss

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
203,302

6.

Shared Voting Power
2,060,432

7.

Sole Dispositive Power
2,667,134

8.

Shared Dispositive Power
-0-

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
2,667,134

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)
7.7%

12.

Type of Reporting Person (See Instructions)
IN

CUSIP No. 015351109

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
Michael L. Sjöström

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
Swiss

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power
203,302

6.

Shared Voting Power
2,060,432

7.

Sole Dispositive Power
2,667,134

8.

Shared Dispositive Power
-0-

9.

Aggregate Amount Beneficially Owned by Each Reporting Person
2,667,134

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)
7.7%

12.

Type of Reporting Person (See Instructions)
IN

Item 1.

(a)

Name of Issuer
Alexion Pharmaceuticals Inc.

(b)

Address of Issuer’s Principal Executive Offices
352 Knotter Drive, Cheshire, CT 06410, USA

Item 2.

(a)

Name of Person Filing
Sectoral Asset Management Inc.

Jérôme G. Pfund

Michael L. Sjöström

(b)

Address of Principal Business Office or, if none, Residence
The principal business address of each person filing is:

2120-1000 Sherbrooke St. West Montreal PQ H3A 3G4 Canada

(c)

Citizenship
Sectoral Asset Management Inc. is a Canadian corporation

Jérôme G. Pfund is a Swiss citizen

Michael L. Sjöström is a Swiss citizen

(d)

Title of Class of Securities
Common Stock

(e)

CUSIP Number
015351109

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)

x

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)

x

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).

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.

Sectoral Asset Management Inc, in its capacity as an investment adviser, has the sole right to dispose of or vote the number of shares of common stock of the Issuer set forth in this filing.  Jérôme G. Pfund and Michael L. Sjöström are the sole shareholders of Sectoral Asset Management Inc.  Sectoral Asset Management, Inc. and Messrs. Pfund and Sjöström disclaim beneficial ownership of the Issuer’s common stock held by Sectoral Asset Management Inc.

(a)

Amount beneficially owned:   

Sectoral Asset Management Inc.:  2,667,134 shares

Jérôme G. Pfund:  2,667,134 shares

Michael L. Sjöström:  2,667,134 shares

(b)

Percent of class:   

Sectoral Asset Management Inc.:  7.7%

Jérôme G. Pfund:  7.7%

Michael L. Sjöström:  7.7%

(c)

Number of shares as to which the person has:

(i)

Sole power to vote or to direct the vote   

Sectoral Asset Management Inc.:  203,302 shares

Jérôme G. Pfund:  203,302 shares

Michael L. Sjöström:  203,302 shares

(ii)

Shared power to vote or to direct the vote    

Sectoral Asset Management Inc.:  2,060,432 shares

Jérôme G. Pfund:  2,060,432 shares

Michael L. Sjöström:  2,060,432 shares

(iii)

Sole power to dispose or to direct the disposition of   

Sectoral Asset Management Inc.:  2,667,134 shares

Jérôme G. Pfund:  2,667,134 shares

Michael L. Sjöström:  2,667,134 shares

(iv)

Shared power to dispose or to direct the disposition of   

Not applicable.

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.

Item 6.

Ownership of More than Five Percent on Behalf of Another Person

Various persons, as investment advisory clients of Sectoral Asset Management, Inc.,  have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, the common stock of the Issuer.  To the knowledge of Sectoral Asset Management, Inc., no one such person’s interest in the common stock of the Issuer is more than five percent of the total outstanding common stock of the Issuer, other than Pictet Funds-BIOTECH, a Luxemburg investment company that beneficially owns 5.9% of the common stock of the Issuer.

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 the undersigned each certifies that, to the best knowledge and belief of the undersigned, the securities referred to above were acquired and are held in the ordinary course of business and 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.

Dated: February 2, 2007

SECTORAL ASSET MANAGEMENT INC.

/s/Jérôme G. Pfund

By: Jérôme G. Pfund

Its: CEO

Dated: February 2, 2007

/s/Jérôme G. Pfund

Jérôme G. Pfund

Dated: February 2, 2007

/s/Michael L. Sjöström

Michael L. Sjöström

SECTORAL ASSET MANAGEMENT INC.

/s/Jérôme G. Pfund

By: Jérôme G. Pfund

Its: CEO

/s/Jérôme G. Pfund

Jérôme G. Pfund

/s/Michael L. Sjöström

Michael L. Sjöström