13G Filing: Perceptive Advisors and CytomX Therapeutics Inc (CTMX)

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Item 1(a).

Name of Issuer:

CytomX Therapeutics, Inc. (the Issuer)

Item 1(b).

Address of Issuers Principal Executive Offices:

151 Oyster Point Blvd.,
Suite 400, South San Francisco, CA 94080

Item 2(a).

Names of Persons Filing:

The names of the persons filing this report
(collectively, the Reporting Persons) are:

Perceptive Advisors LLC (Perceptive Advisors)

Joseph Edelman (Mr. Edelman)

Perceptive Life Sciences Master Fund, Ltd. (the Master Fund)

Item 2(b).

Address of Principal Business Office or, if None, Residence:

The address of the
principal business office of each of the Reporting Persons is:

51 Astor Place, 10th Floor

New York, NY 10003

Item 2(c).

Citizenship:

Perceptive Advisors is a Delaware limited liability company

Mr. Edelman is a United States citizen

The Master Fund is a Cayman Islands corporation

Item 2(d).

Title of Class of Securities:

Common Stock, $0.00001 par
value (Common Stock)

Item 2(e).

CUSIP Number:

23284F 10 5

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:

Not applicable.

Item 4.

Ownership.

The information required by this item with respect to each Reporting
Person is set forth in Rows 5 through 9 and 11 of the cover page to this Schedule 13G. The ownership percentages reported are based on 38,408,626 outstanding shares of Common Stock, as reported in the Issuers Form 10-Q filed on November 7, 2017.