Tax-driven share sale by CytomX (CTMX) chief scientist on RSU vesting

Rhea-AI Filing Summary

CytomX Therapeutics, Inc. executive Marcia Belvin, SVP and Chief Scientific Officer, reported an open-market sale of 31,492 shares of common stock on March 17, 2026 at $6.423 per share. The sale was made solely to satisfy tax or other government withholding obligations tied to vesting restricted stock units (RSUs).

After this transaction, Belvin directly holds 300,760 shares of common stock, which includes 135,833 RSUs. This filing reflects a compensation-related tax obligation rather than a discretionary portfolio trade.

SEC Form 4

FORM 4

UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934
or Section 30(h) of the Investment Company Act of 1940

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Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

Check this box to indicate that a transaction was made pursuant to a contract, instruction or written plan for the purchase or sale of equity securities of the issuer that is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). See Instruction 10.

1. Name and Address of Reporting Person^*

BELVIN MARCIA

(Last) (First) (Middle)

C/O CYTOMX THERAPEUTICS, INC.

151 OYSTER POINT BLVD., STE. 400

(Street)

SOUTH SAN FRANCISCO CALIFORNIA 94080

(City) (State) (Zip)

UNITED STATES

(Country)

2. Issuer Name and Ticker or Trading Symbol
CytomX Therapeutics, Inc. [ CTMX ]

5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)

	Director		10% Owner
X	Officer (give title below)		Other (specify below)
	SVP, Chief Scientific Officer

2a. Foreign Trading Symbol

3. Date of Earliest Transaction (Month/Day/Year)
03/17/2026

6. Individual or Joint/Group Filing (Check Applicable Line)

X	Form filed by One Reporting Person
	Form filed by More than One Reporting Person

4. If Amendment, Date of Original Filed (Month/Day/Year)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1. Title of Security (Instr. 3)	2. Transaction Date (Month/Day/Year)	2A. Deemed Execution Date, if any (Month/Day/Year)	3. Transaction Code (Instr. 8)		4. Securities Acquired (A) or Disposed Of (D) (Instr. 3, 4 and 5)			5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4)	6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	7. Nature of Indirect Beneficial Ownership (Instr. 4)
			Code	V	Amount	(A) or (D)	Price
Common Stock	03/17/2026		S(1)		31,492	D	$6.423	300,760(2)	D

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 3)	2. Conversion or Exercise Price of Derivative Security	3. Transaction Date (Month/Day/Year)	3A. Deemed Execution Date, if any (Month/Day/Year)	4. Transaction Code (Instr. 8)		5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5)		6. Date Exercisable and Expiration Date (Month/Day/Year)		7. Title and Amount of Securities Underlying Derivative Security (Instr. 3 and 4)		8. Price of Derivative Security (Instr. 5)	9. Number of derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)	10. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)
				Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares

Explanation of Responses:

1. The shares were sold solely to satisfy tax or other government withholding obligations in connection with the vesting of restricted stock units ("RSUs").

2. Includes 135,833 RSUs.

	/s/ Lloyd Rowland, as Attorney-in-Fact for Marcia Belvin	03/19/2026
	** Signature of Reporting Person	Date
Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
* If the form is filed by more than one reporting person, see Instruction 4 (b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.
* Form 4: SEC 1474 (03-26)

FAQ

What insider transaction did CytomX (CTMX) report for Marcia Belvin?

CytomX reported that SVP and Chief Scientific Officer Marcia Belvin sold 31,492 shares of common stock. The shares were sold in the open market to satisfy tax or other government withholding obligations related to the vesting of restricted stock units.

How many CytomX (CTMX) shares did Marcia Belvin sell and at what price?

Marcia Belvin sold 31,492 shares of CytomX common stock at $6.423 per share. The transaction occurred on March 17, 2026, and was executed as an open-market sale according to the Form 4 disclosure.

Why were Marcia Belvin’s CytomX (CTMX) shares sold in this Form 4 filing?

The shares were sold solely to satisfy tax or other government withholding obligations connected with the vesting of restricted stock units. This indicates the sale was driven by compensation-related tax requirements rather than a discretionary investment decision.

How many CytomX (CTMX) shares does Marcia Belvin hold after this sale?

Following the reported sale, Marcia Belvin directly holds 300,760 shares of CytomX common stock. This total includes 135,833 restricted stock units, which represent additional share-based compensation that vests over time under company equity plans.

What are RSUs mentioned in the CytomX (CTMX) insider filing?

RSUs, or restricted stock units, are share-based awards that convert into common stock as they vest over time. The filing notes that Marcia Belvin’s holdings include 135,833 RSUs, reflecting a significant component of her equity compensation at CytomX.

Does the CytomX (CTMX) insider sale indicate a change in company outlook?

The filing describes the sale as solely to cover tax or other government withholding obligations from RSU vesting. Such transactions are typically administrative, reflecting compensation-related tax payments rather than a change in sentiment about CytomX’s business prospects.