Sec Form 4 Filing - Mandal Sumant @ RUBICON PROJECT, INC. - 2019-02-25

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

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, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Mandal Sumant 2. Issuer Name and Ticker or Trading Symbol RUBICON PROJECT, INC. [ RUBI] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director _____ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 725 ARIZONA AVENUE, SUITE 304 3. Date of Earliest Transaction (MM/DD/YY) 02/25/2019 (Street) SANTA MONICA, CA90401 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 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 02/25/2019 J( 1 )( 2 ) 1,746 A $ 0 ( 1 ) ( 2 ) 44,988 ( 1 ) D Common Stock 02/25/2019 J( 2 )( 3 ) 508,938 D $ 0 ( 2 ) ( 3 ) 1,548,980 ( 2 ) ( 3 ) I See Footnotes ( 4 ) ( 5 )

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 (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 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 (MM/DD/YY) 7. Title and Amount of Underlying Securities (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 of Derivative Security: 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

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Mandal Sumant 725 ARIZONA AVENUE SUITE 304 SANTA MONICA, CA90401 X Signatures /s/ Jonathan Feldman, attorney-in-fact 02/26/2019 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Reflects 1,746 shares of Common Stock of the Issuer (the "Shares") received by the reporting person on February 25, 2019 in connection with pro-rata distributions-in-kind of shares for no consideration. The aforementioned distributions were made in accordance with the exemptions afforded by Rule 16a-13 and Rule 16a-9 of the Securities Exchange Act of 1934, as amended. ( 2 )Represents the following distributions on February 25, 2019, for no consideration: (i) Clearstone Venture Partners III-A, L.P., a Delaware limited partnership ("CVP III-A"), distributed 500,000 Shares to its partners, including 1,241 Shares to its General Partner, Clearstone Venture Management III, L.L.C., a Delaware limited liability company ("CVM III"), representing each such partners' pro rata interest in the Shares held by CVP III-A, and (ii) Clearstone Venture Partners III-B, L.P., a Delaware multiple series limited liability company ("CVP III-B"), distributed 8,938 Shares to its members, including the reporting person, representing each such members' pro rata interest in the Shares held by CVP III-B. On February 25, 2019, CVM III distributed to its members, including the reporting person, the 1,241 Shares that it received as a distribution on February 25, 2019 from CVP III-A. ( 3 )These distributions were made in accordance with the exemptions afforded by Rule 16a-13 and Rule 16a-9 of the Securities Exchange Act of 1934, as amended. ( 4 )Represents (i) 11,283 Shares directly beneficially owned by CVM III, (ii) 1,510,692 Shares directly beneficially owned by CVP III-A, and (iii) 27,005 Shares directly beneficially owned by CVP III-B (together with CVM III and CVP III-A, the "Funds"). The reporting person is a managing member of CVM III and a member of CVP III-B. ( 5 )The reporting person disclaims beneficial ownership of the Shares held by the Funds, except to the extent of his pecuniary interest therein, if any, and the inclusion of these Shares in this report shall not be deemed an admission of beneficial ownership of the reported shares for purposes of Section 16 or for any other purpose.

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