460 WARD DRIVE, SUITE E1 SANTA BARBARA, CA 93111 TEL ###-###-#### FAX ###-###-#### WWW.CYTOMX.COM Revised December 15, 2010

Exhibit 10.7

460 WARD DRIVE, SUITE E1

SANTA BARBARA, CA 93111

TEL 805 ###-###-####    FAX 805 ###-###-####

WWW.CYTOMX.COM

Revised December 15, 2010

Sean A. McCarthy, D. Phil.

Dear Dr. McCarthy:

On behalf of CYTOMX THERAPEUTICS, INC., a Delaware corporation (the “Company”), I am pleased to write to confirm the revised terms and conditions of our offer of employment with the Company as set forth below, which shall commence on December 30, 2010. Initially you will work fifty (50) percent of your business time and efforts during normal business hours for the Company, and be paid accordingly. Commencing February 1, 2011, you will work for the Company full time and be paid full time.

The terms and conditions of your employment will be as follows:

[Signatures appear on the following page.]

ACCEPTANCE

Please contact me if you have any questions regarding this letter agreement, the Company, its policies or procedures. If the terms and conditions set forth above are acceptable to you, then please sign and return to me one copy of this letter agreement for our files.

We look forward to working with you.

ACCEPTANCE

The undersigned agrees to and accepts the terms and conditions set forth above.

EXHIBIT A

Employer: CytomX Therapeutics, Inc.

Employee: Sean A. McCarthy, D. Phil.

Certain Defined Terms

For purposes of this offer letter, a “Change of Control” shall mean the occurrence of any of the following events, provided that such event or occurrence constitutes a change in the ownership or effective control of the Company, or a change in the ownership of a substantial portion of the assets of the Company, as defined in Treasury Regulation §§ 1.409A-3(i)(5)(v), (vi), and (vii): (i) any merger or consolidation that results in the voting securities of the Company outstanding immediately prior thereto representing (either by remaining outstanding or by being converted into voting securities of the surviving or acquiring entity) less than 50% of the combined voting power of the voting securities of the Company or such surviving or acquiring entity outstanding immediately after such merger or consolidation; (ii) any sale of all or substantially all of the assets of the Company; (iii) the complete liquidation or dissolution of the Company; or (iv) the acquisition of “beneficial ownership” (as defined in Rule 13d-3 under the Exchange Act) of securities of the Company representing 50% or more of the combined voting power of the Company’s then outstanding securities (other than through a merger or consolidation or an acquisition of securities directly from The Company) by any “person,” as such term is used in Sections 13(d) and 14(d) of the Exchange Act, other than the Company, any trustee or other fiduciary holding securities under an employee benefit plan of the Company or any corporation owned directly or indirectly by the stockholders of the Company in substantially the same proportion as their ownership of stock of the Company.

For purposes of this offer letter, “Cause” shall mean (i) a material breach of any material term of this offer letter or the nondisclosure and assignment of inventions agreement between you and the Company, (ii) a plea of guilty or nolo contendere to, or conviction of, the commission of a felony offense, (iii) repeated unexplained or unjustified absence, or refusals to carry out the lawful directions of the Board or (iv) material breach of a fiduciary duty owed to the Company, provided that any action or inaction described by (i), (iii) or (iv), above, shall not be the basis of a termination of your employment with the Company for “Cause” unless the Company provides you with at least 20 days advance written notice specifying in reasonable detail the conduct in need of being cured, provides you with the opportunity to address the Board regarding such actions or inactions and such conduct was not cured within the notice period or prior to termination.

For purposes of this offer letter, “Good Reason” shall mean the occurrence of any of the following events or circumstances (i) a material diminution in your authority or duties without your prior consent; (ii) a material breach by the Company of the terms of this offer letter; or (iii) the relocation of the Company’s headquarters to a place outside of a 45-mile radius from the company’s headquarters in San Francisco, California (provided that for the avoidance of doubt, the relocation of such headquarters from Santa Barbara, California, to San Francisco, California, shall not provide a basis for “Good Reason” hereunder). In order to establish a “Good Reason” for terminating employment, you must provide written notice to the Company of the existence of the condition giving rise to the Good Reason, which notice must be provided within 15 days of the initial existence of such condition, the Company must fail to cure the condition within 30 days thereafter, and your termination of employment must occur no later than seven (7) days following the expiration of that 30-day cure period.

EXHIBIT B

Employer: CytomX Therapeutics, Inc.

Employee: Sean A. McCarthy, D. Phil.

Payments Subject to Section 409A

The following rules shall apply with respect to distribution of the payments and benefits, if any, to be provided under the attached offer letter:

(a) It is intended that each installment of the severance or change of control payments and benefits provided under the offer letter shall be treated as a separate “payment” for purposes of Section 409A of the Internal Revenue Code and the guidance issued thereunder (“Section 409A”). Neither you nor the Company shall have the right to accelerate or defer the delivery of any such payments or benefits except to the extent specifically permitted or required by Section 409A.

(b) If, as of the date of your “separation from service” from the Company, you are not a “specified employee” (within the meaning of Section 409A), then each installment of the severance payments and benefits shall be made on the dates and terms set forth in this offer letter.

(c) If, as of the date of your “separation from service” from the Company, you are a “specified employee” (within the meaning of Section 409A), then:

(i) Each installment of the severance or change of control payments and benefits due under this offer letter that, in accordance with the dates and terms set forth herein, will in all circumstances, regardless of when the separation from service occurs, be paid within the short-term deferral period (as defined under Section 409A) shall be treated as a short-term deferral within the meaning of Treasury Regulation Section 1.409A-1(b)(4) to the maximum extent permissible under Section 409A; and

(ii) Each installment of the severance or change of control payments and benefits due under this offer letter that is not described in paragraph (i) above and that would, absent this subsection, be paid within the six-month period following your “separation from service” from The Company shall not be paid until the date that is six months and one day after such separation from service (or, if earlier, upon your death), with any such installments that are required to be delayed being accumulated during the six-month period and paid in a lump sum on the date that is six months and one day following your separation from service and any subsequent installments, if any, being paid in accordance with the dates and terms set forth herein; provided, however, that the preceding provisions of this sentence shall not apply to any installment of severance or change of control payments and benefits if and to the maximum extent that that such installment is deemed to be paid under a separation pay plan that does not provide for a deferral of compensation by reason of the application of Treasury Regulation Section 1.409A-1(b)(9)(iii) (relating to separation pay upon an involuntary separation from service). Any installments that qualify for the exception under Treasury Regulation Section 1.409A-1(b)(9)(iii) must be paid no later than the last day of the Employee’s second taxable year following the taxable year in which the separation from service occurs.

(d) The determination of whether and when your separation from service from the Company has occurred shall be made and in a manner consistent with and based on the presumptions set forth in, Treasury Regulation Section 1.409A-1(h), Solely for purposes of this paragraph (d), “the Company” shall include all persons with whom the Company would be considered a single employer under Sections 414(b) and 414(c) of the Code.

(e) All reimbursements and in-kind benefits provided under this offer letter shall be made or provided in accordance with the requirements of Section 409A to the extent that such reimbursements or in-kind benefits are subject to Section 409A, including, where applicable, the requirement that (i) any reimbursement is for expenses incurred during your lifetime (or during a shorter period of time specified in this offer letter), (ii) the amount of expenses eligible for reimbursement during a calendar year may not affect the expenses eligible for reimbursement in any other calendar year, (iii) the reimbursement of an eligible expense will be made on or before the last day of the calendar year following the year in which the expense is incurred and (iv) the right to reimbursement is not subject to set off or liquidation or exchange for any other benefit.

Exhibit B, Page 2

FIRST AMENDMENT TO OFFER LETTER

THIS FIRST AMENDMENT TO OFFER LETTER (the “Amendment”) is made and entered into, effective as of August 9, 2011 (the “Effective Date”), by and between CYTOMX THERAPEUTICS, INC. a Delaware corporation (the “Company”), and SEAN MCCARTHY (“Employee”), with reference to the following facts:

RECITALS:

A. The parties previously executed that certain letter agreement dated December 15, 2010, “Re: Offer of Employment” (the “Offer Letter”), pursuant to which the Company employs Employee as its Chief Business Officer.

B. The parties have agreed to execute this Amendment in order to memorialize the appointment of Employee as Chief Executive Officer of the Company, an increase in Employee’s base compensation and target bonus percentage, and certain agreements regarding Employee’s equity compensation.

AGREEMENTS:

NOW, THEREFORE, the parties hereto, intending to be legally bound, do hereby agree as follows:

1. AMENDMENT OF OFFER LETTER. The Offer Letter is hereby amended as follows.

1.1 POSITION, TITLE, AND DUTIES. The paragraph entitled “Title, Position and duties” is hereby amended and replaced in its entirety with the following:

“Title, Position and Duties. You will hold the position of Chief Executive Officer with the Company, and you will report to the Company’s Board of Directors. In this capacity, you shall perform such customary, appropriate and reasonable executive duties as are usually performed by the Chief Executive Officer of an operating company, including such duties as are delegated to you from time to time by the Board of Directors of the Company.”

1.2 BASE COMPENSATION AND ANNUAL BONUS. The paragraphs labeled “Base Compensation” and “Annual Bonus” are hereby amended and replaced in their entirety with the following:

“Base Compensation. You will be paid Base Salary at the annual rate of Three Hundred Thirty-five Thousand Dollars ($335,000) per year, payable in equal periodic installments in accordance with the Company’s payroll practices in effect from time to time.

Annual Bonus. You shall be eligible to receive an annual cash bonus in an amount of up to twenty-five percent (25%) of your Base Salary then in effect for each calendar year, less withholding required by law, based upon achievement of such performance criteria as are established by the Board following prior consultation with you. The performance criteria for each calendar year shall be determined by the Company prior to, or promptly following the commencement of each calendar year, and shall be communicated to you in writing. The maximum percentage of such annual bonus shall be reviewed and may be increased from time to time with the approval of the Board.

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1.3 EQUITY COMPENSATION. During the period following the Effective Date of this Amendment, the Company and Employee shall negotiate the terms on which the Company shall grant to Employee, under the Company’s 2010 Equity Incentive Plan, an option to purchase additional shares of the Company’s common stock, which option (a) shall provide for (i) vesting in a portion of the shares subject to that option, commencing as of the Effective Date of this Amendment, based upon the customary four-year vesting schedule under the Company’s 2010 Equity Incentive Plan, and (ii) vesting in the remainder of the shares subject to that option based upon the achievement of such performance milestones as are set forth in the option agreement, and (b) shall be presented to the Board of Directors for approval at the meeting of the Board scheduled to be held on September 14, 2011.

2. MISCELLANEOUS. Except as expressly modified by Section 1, above, the Offer Letter is hereby ratified and confirmed and shall remain in full force and effect. This Agreement may be executed in counterparts, each of which shall be deemed an original and both of which, taken together, shall constitute one and the same instrument, binding on each signatory thereto. A copy of this Amendment that is executed by a party and transmitted by that party to the other party by facsimile or as an attachment (e.g., in “.tif” or “.pdf” format) to an email shall be binding upon the signatory to the same extent as a copy hereof containing that party’s original signature.

[Signatures appear on the following page.]

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IN WITNESS WHEREOF, the undersigned have executed this Amendment, effective as of the Effective Date set forth above.

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IN WITNESS WHEREOF, the undersigned have executed this Amendment, effective as of the Effective Date set forth above,

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