Sec Form 3 Filing - Atlas Venture Fund VIII, L.P. @ INTEGRAL AD SCIENCE HOLDING CORP. - 2021-06-29

FORM 3 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 Atlas Venture Fund VIII, L.P. 2. Issuer Name and Ticker or Trading Symbol INTEGRAL AD SCIENCE HOLDING CORP. [ IAS] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 56 WAREHAM STREET, FLOOR 3 3. Date of Earliest Transaction (MM/DD/YY) 06/29/2021 (Street) BOSTON, MA02118 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ 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, $0.001 par value 22,722,770 I See Footnote ( 1 )

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 Atlas Venture Fund VIII, L.P. 56 WAREHAM STREET, FLOOR 3 BOSTON, MA0211 8 X Atlas Venture Associates VIII, L.P. 56 WAREHAM STREET, FLOOR 3 BOSTON, MA02118 X Atlas Venture Associates VIII, Inc. 56 WAREHAM STREET, FLOOR 3 BOSTON, MA02118 X Signatures Atlas Venture Fund VIII, L.P., By: Atlas Venture Associates VIII, L.P., its general partner, By: Atlas Venture Associates VIII, Inc., its general partner, By: Frank Castellucci, General Counsel and Secretary /s/ Frank Castellucci 06/29/2021 Signature of Reporting Person Date Atlas Venture Associates VIII, L.P. By: Atlas Venture Associates VIII, Inc., its general partner, By: Frank Castellucci, General Counsel and Secretary /s/ Frank Castellucci 06/29/2021 Signature of Reporting Person Date Atlas Venture Associates VIII, Inc, By: Frank Castellucci, General Counsel and Secretary /s/ Frank Castellucci 06/29/2021 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Includes 22,722,770 shares held directly by Atlas Venture Fund VIII, L.P. ("Atlas VIII"). Atlas Venture Associates VIII, L.P. ("AVA VIII LP") is the sole general partner of Atlas VIII. Atlas Venture Associates VIII, Inc. ("AVA VIII Inc.") is the sole general partner of AVA VIII LP. Each of Atlas VIII, AVAI VIII LP and AVA VIII Inc. disclaim beneficial ownership of all shares except to the extent of its pecuniary interest, if any, therein. This report shall not be deemed to be an admission that the reporting persons are the beneficial owners of such securities for purposes of Section 16 or for any other purpose.

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).

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