13G Filing: Biotechnology Value Fund LP and CytomX Therapeutics Inc (CTMX)

 UNITED STATES

SECURITIES AND EXCHANGE
COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

(Rule 13d-102)

INFORMATION TO BE INCLUDED IN STATEMENTS FILED PURSUANT

TO RULES 13d-1(b), (c),
AND (d) AND AMENDMENTS THERETO FILED

PURSUANT TO RULE 13d-2(b)

(Amendment No.
)¹

Check the appropriate box
to designate the rule pursuant to which this Schedule is filed:

CytomX Therapeutics, Inc., a Delaware corporation (the “Issuer”).

151 Oyster Point Boulevard, Suite 400

South San Francisco, California 94080

Biotechnology Value Fund, L.P. (“BVF”)

1 Sansome Street, 30^th Floor

San Francisco, California 94104

Citizenship: Delaware

Biotechnology Value Fund II, L.P. (“BVF2”)

1 Sansome Street, 30^th Floor

San Francisco, California 94104

Citizenship: Delaware

Biotechnology Value Trading Fund OS
LP (“Trading Fund OS”)

PO Box 309 Ugland House

Grand Cayman, KY1-1104

Cayman Islands

Citizenship: Cayman Islands

BVF Partners OS Ltd. (“Partners
OS”)

PO Box 309 Ugland House

Grand Cayman, KY1-1104

Cayman Islands

Citizenship: Cayman Islands

BVF Partners L.P. (“Partners”)

1 Sansome Street, 30th Floor

San Francisco, California 94104

Citizenship: Delaware

BVF Inc.

1 Sansome Street, 30^th Floor

San Francisco, California 94104

Citizenship: Delaware

Mark N. Lampert (“Mr. Lampert”)

1 Sansome Street, 30^th Floor

San Francisco, California 94104

Citizenship: United States

Each of the foregoing is referred to
as a “Reporting Person” and collectively as the “Reporting Persons.”

Common Stock, par value $0.0001 per share (the “Common
Stock”)

23284F 10 5

As of the close of business on June
29, 2017 (i) BVF beneficially owned 941,783 Common Stock, (ii) BVF2 beneficially owned 608,749 Common Stock, and (iii) Trading
Fund OS beneficially owned 171,334 Common Stock.

Partners OS as the general partner
of Trading Fund OS may be deemed to beneficially own the 171,334 Common Stock beneficially owned by Trading Fund OS.

Partners, as the general partner of
BVF, BVF2, the investment manager of Trading Fund OS, and the sole member of Partners OS, may be deemed to beneficially own the
2,026,998 Common Stock beneficially owned in the aggregate by BVF, BVF2, Trading Fund OS, and a certain Partners managed accounts
(the “Partners Managed Accounts”), including 305,132 Common Stock held in the Partners Managed Accounts.

BVF Inc., as the general partner of
Partners, may be deemed to beneficially own the 2,026,998 Common Stock beneficially owned by Partners.

Mr. Lampert, as a director and officer
of BVF Inc., may be deemed to beneficially own the 2,026,998 Common Stock beneficially owned by BVF Inc.

The foregoing should not be construed
in and of itself as an admission by any Reporting Person as to beneficial ownership of any Common Stock owned by another Reporting
Person. Partners OS disclaims beneficial ownership of the Common Stock beneficially owned by Trading Fund OS. Each of Partners,
BVF Inc. and Mr. Lampert disclaims beneficial ownership of the Common Stock beneficially owned by BVF, BVF2, Trading Fund OS, and
the Partners Managed Accounts, and the filing of this statement shall not be construed as an admission that any such person or
entity is the beneficial owner of any such securities.

The following percentages are based
on 36,749,732 shares of Common Stock outstanding, as of April 28, 2017, which is the total number of shares of Common Stock outstanding
as reported in the Issuer’s Definitive Proxy Statement, filed with the Securities and Exchange Commission on May 9, 2017.

As of the close of business on June
29, 2017 (i) BVF beneficially owned approximately 2.6% of the outstanding Common Stock, (ii) BVF2 beneficially owned approximately
1.7% of the outstanding Common Stock, (iii) Trading Fund OS beneficially owned less than 1% of the outstanding Common Stock (iv)
Partners OS may be deemed to beneficially own less than 1% of the outstanding Common Stock, and (v) each of Partners, BVF Inc.
and Mr. Lampert may be deemed to beneficially own approximately 5.5% of the outstanding Common Stock (less than 1% of which is
held in the Partners Managed Accounts).

See Cover Pages Items 5-9.

See Cover Pages Items 5-9.

See Cover Pages Items 5-9.

See Cover Pages Items 5-9.

Not Applicable.

Partners, BVF Inc. and Mr. Lampert
share voting and dispositive power over the shares of Common Stock beneficially owned by BVF, BVF2, Trading Fund OS, and the Partners
Managed Accounts.

Not Applicable.

See Exhibit 99.1.

Not Applicable.

By signing below each of
the undersigned certifies that, to the best of its 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.

SIGNATURE

After reasonable inquiry
and to the best of his knowledge and belief, each of the undersigned certifies that the information set forth in this statement
is true, complete and correct.

Dated: June 29, 2017