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             Filed by the Registrant ☒ 
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             Filed by a Party other than the Registrant  ☐ 
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               Preliminary Proxy Statement 
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                ☐ 
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               Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2)) 
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| ☐ | Definitive Proxy Statement  | 
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| ☒ | Definitive Additional Materials  | 
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| ☐ | Soliciting Material Pursuant to § 240.14a-12 | 
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             CytomX Therapeutics, Inc. 
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             (Name of Registrant as Specified In Its Charter) 
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             (Name of Person(s) Filing Proxy Statement, if other than the Registrant) 
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             No fee required. 
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              ☐ 
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             Fee paid previously with preliminary materials. 
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| ☐ | Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 14a-6(i)(1) and 0-11.  | 
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