Sec Form 4 Filing - GGC Opportunity Fund Management GP, Ltd. @ Cornerstone Building Brands, Inc. - 2021-03-03

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 GGC Opportunity Fund Management GP, Ltd. 2. Issuer Name and Ticker or Trading Symbol Cornerstone Building Brands, Inc. [ CNR] 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) C/O GOLDEN GATE PRIVATE EQUITY, INC., ONE EMBARCADERO CENTER 39TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 03/03/2021 (Street) SAN FRANCISCO, CA94111 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, $0.01 par value 03/03/2021 S 150,000 D $ 11.62 ( 1 ) 1,404,586 I See notes ( 2 ) ( 3 ) Common Stock, $0.01 par value 03/04/2021 S 277,800 D $ 12.31 ( 4 ) 1,126,786 I See notes ( 2 ) ( 3 ) Common Stock, $0.01 par value 03/05/2021 S 385,000 D $ 12.52 ( 5 ) 741,786 I See notes ( 2 ) ( 3 ) Common Stock, $0.01 par value 12,441,252.4 I See note ( 6 ) Common Stock, $0.01 par value 48,150.31 I See note ( 7 )

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 GGC Opportunity Fund Management GP, Ltd. C/O GOLDEN GATE PRIVATE EQUITY, INC. ONE EMBARCADERO CENTER 39TH FLOOR SAN FRANCISCO, CA94111 X GGCOF EXECUTIVE CO-INVEST, L.P. C/O GOLDEN GATE PRIVATE EQUITY, INC. ONE EMBARCADERO CENTER, 39TH FLOOR SAN FRANCISCO, CA94111 X GGCOF IRA CO-INVEST, L.P. C/O GOLDEN GATE PRIVATE EQUITY, INC. ONE EMBARCADERO CENTER, 39TH FLOOR SAN FRANCISCO, CA94111 X Signatures GGC Opportunity Fund Management GP, Ltd., By: /s/ Stephen D. Oetgen, attorney-in-fact for David Dominik, a Director 03/05/2021 Signature of Reporting Person Date GGC Atrium Window Holdings, LLC, By: /s/ Stephen D. Oetgen, attorney-in-fact for David Thomas, a Manager 03/05/2021 Signature of Reporting Person Date GGC BP Holdings, LLC, By: /s/ Stephen D. Oetgen, attorney-in-fact for David Thomas, a Manager 03/05/2021 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Weighted average price representing high of $11.78 and low of $11.50. ( 2 )Shares are directly held by AIC Finance Partnership, L.P. ("AIC"). Each of Golden Gate Capital Opportunity Fund, L.P. ("GGCOF"), Golden Gate Capital Opportunity Fund-A, L.P. ("GGCOF-A"), GGCOF Co-Invest, L.P. ("GGCOF Co-Invest"), GGCOF Co-Invest IRA, L.P. ("IRA Co-Invest") and GGCOF Executive Co-Invest, L.P. ("Executive Co-Invest" and, together with GGCOF, GGCOF-A, GGCOF Co-Invest and IRA Co-Invest, the "Funds") have an indirect pecuniary interest in AIC. GGCOF Co-Invest Management, L.P. ("GGCOF Third Party Co-Invest") is the general partner of each of GGCOF Co-Invest, IRA Co-Invest and Executive Co-Invest. GGC Opportunity Fund Management, L.P. ("Management GP") is the general partner of each of GGCOF, GGCOF-A and GGCOF Third Party Co-Invest, and GGC Opportunity Fund Management GP, Ltd. ( 3 )(Continued from Footnote 3) ("Ultimate GP") is the general partner of Management GP. Ultimate GP is governed by its board of directors. Each of the Funds, GGCOF Third Party Co-Invest, Management GP and Ultimate GP disclaim beneficial ownership of these shares except to the extent of their respective pecuniary interests therein. ( 4 )Weighted average price representing high of $12.78 and low of $11.80. ( 5 )Weighted average price representing high of $12.82 and low of $12.15. ( 6 )Shares are directly held by Atrium Intermediate Holdings, LLC ("Atrium Intermediate"). Atrium Window Holdings, LLC ("Atrium Holdings") and Atrium Window Parent, LLC ("Atrium Parent") are the members of Atrium Intermediate. Atrium Parent is the controlling unitholder of Atrium Holdings. GGC Atrium Window Holdings, LLC ("GGC Atrium') is the controlling unitholder of Atrium Parent. GGC BP Holdings, LLC ("GGC BP") is the controlling unitholder of GGC Atrium. The Funds collectively hold all of the equity interests of GGC BP. Each of Funds, GGCOF Third Party Co-Invest, Management GP and Ultimate GP disclaim beneficial ownership of these shares except to the extent of their respective pecuniary interests therein. ( 7 )Shares are directly held by GGC BP. Each of the Funds, GGCOF Third Party Co-Invest, Management GP and Ultimate GP disclaim beneficial ownership of these shares except to the extent of their respective pecuniary interests therein.

Remarks: Form 2 of 2

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