Sec Form 4 Filing - MGC Venture Partners 2013 GP, LLC @ CASTLE BIOSCIENCES INC - 2019-07-29

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 MGC Venture Partners 2013 GP, LLC 2. Issuer Name and Ticker or Trading Symbol CASTLE BIOSCIENCES INC [ CSTL] 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 MOUNTAIN GROUP CAPITAL, LLC, 3835 CLEGHORN AVE., SUITE 300 3. Date of Earliest Transaction (MM/DD/YY) 07/29/2019 (Street) NASHVILLE, TN37215 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 07/29/2019 C 1,336,927 ( 1 ) A 1,336,927 I See footnotes ( 3 ) ( 4 ) ( 5 ) Common Stock 07/29/2019 C 97,428 A $ 12.8 ( 6 ) 1,434,355 I See footnotes ( 3 ) ( 4 ) ( 5 ) Common Stock 07/29/2019 P 62,500 ( 7 ) A $ 16 1,496,855 I See footnotes ( 3 ) ( 4 ) ( 5 ) Common Stock 07/29/2019 P 115,300 ( 7 ) A $ 16 115,300 I See footnotes ( 4 ) ( 5 ) ( 8 ) Common Stock 07/29/2019 P 134,700 ( 7 ) A $ 16 134,700 I See footnotes ( 4 ) ( 5 ) ( 9 )

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. Transac tion 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 Series A Preferred Stock ( 2 ) 07/29/2019 C 19,524 ( 2 ) ( 2 ) CommonStock 16,015 ( 2 ) 0 I See footnotes ( 3 ) ( 4 ) ( 5 ) Series D Preferred Stock ( 2 ) 07/29/2019 C 337,754 ( 2 ) ( 2 ) CommonStock 277,074 ( 2 ) 0 I See footnotes ( 3 ) ( 4 ) ( 5 ) Series E-1 Preferred Stock ( 2 ) 07/29/2019 C 463,453 ( 2 ) ( 2 ) CommonStock 380,190 ( 2 ) 0 I See footnotes ( 3 ) ( 4 ) ( 5 ) Series E-2 Preferred Stock ( 2 ) 07/29/2019 C 158,379 ( 2 ) ( 2 ) CommonStock 129,925 ( 2 ) 0 I See footnotes ( 3 ) ( 4 ) ( 5 ) Series E-3 Preferred Stock ( 2 ) 07/29/2019 C 142,843 ( 2 ) ( 2 ) CommonStock 117,180 ( 2 ) 0 I See footnotes ( 3 ) ( 4 ) ( 5 ) Series F Preferred Stock ( 2 ) 07/29/2019 C 507,762 ( 2 ) ( 2 ) CommonStock 416,537 ( 2 ) 0 I See footnotes ( 3 ) ( 4 ) ( 5 ) Convertible Promissory Note $ 12.8 ( 6 ) 07/29/2019 C 1,200,000 ( 6 ) ( 6 ) ( 6 ) Common Stock 97,428 ( 6 ) 0 I See footnotes ( 3 ) ( 4 ) ( 5 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other MGC Venture Partners 2013 GP, LLC C/O MOUNTAIN GROUP CAPITAL, LLC 3835 CLEGHORN AVE., SUITE 300 NASHVILLE, TN37215 X MGC Venture Partners 2013, L.P. C/O MOUNTAIN GROUP CAPITAL, LLC 3835 CLEGHORN AVE., SUITE 300 NASHVILLE, TN37215 X MGC Venture Partners 2018 GP, LLC C/O MOUN TAIN GROUP CAPITAL, LLC 3835 CLEGHORN AVE., SUITE 300 NASHVILLE, TN37215 X MGC VENTURE PARTNERS 2018, L.P. C/O MOUNTAIN GROUP CAPITAL, LLC 3835 CLEGHORN AVE., STE. 300 NASHVILLE, TN37215 X MGC VENTURE PARTNERS QP 2018, L.P. C/O MOUNTAIN GROUP CAPITAL, LLC 3835 CLEGHORN AVE., STE. 300 NASHVILLE, TN37215 X Signatures /s/ Byron W. Smith, Managing Member 2013 GP, LLC, the General Partner of MGC Venture Partners 2013, L.P. 07/29/2019 Signature of Reporting Person Date /s/ Byron W. Smith, Managing Member of MGC Venture Partners 2013 GP, LLC 07/29/2019 Signature of Reporting Person Date /s/ Jason S. Ferguson, Managing Partner of MGC Venture Partners 2018 GP, LLC, the General Partner of MGC Venture Partners 2018, L.P. 07/29/2019 Signature of Reporting Person Date /s/ Jason S. Ferguson, Managing Partner of MGC Venture Partners 2018 GP, LLC, the General Partner of MGC Venture Partners QP 2018, L.P. 07/29/2019 Signature of Reporting Person Date /s/ Jason S. Ferguson, Managing Partner of MGC Venture Partners 2018 GP, LLC 07/29/2019 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The shares of Common Stock issued upon conversion of the Preferred Stock were calculated by first aggregating all series of Preferred Stock held by MGC Venture Partners 2013, L.P. ("MGC 2013 LP"), in accordance with the terms of the then-effective Certificate of Incorporation of Castle Biosciences, Inc. (the "Issuer"). ( 2 )Every 1.219 outstanding shares of Preferred Stock automatically converted into 1 share of Common Stock immediately prior to the closing of the Issuer's initial public offering (the "IPO"), for no additional consideration, with cash paid for any fractional shares. The Preferred Stock had no expiration date. ( 3 )Held directly by MGC 2013 LP. ( 4 )MGC Venture Partners 2013 GP, LLC ("MGC 2013 GP") is the general partner of MGC 2013 LP. MGC Venture Partners 2018 GP, LLC ("MGC 2018 GP") is the general partner of MGC Venture Partners 2018, L.P. ("MGC 2018 LP") and MGC Venture Partners QP 2018, L.P. ("MGC QP 2018 LP"). Each of MGC 2013 LP, MGC 2013 GP, MGC 2018 LP, MGC QP 2018 LP and MGC 2018 GP directly (whether through ownership or position) or indirectly through one or more intermediaries, may be deemed for purposes of Section 16 of the Securities Exchange Act of 1934, as amended, to be the indirect beneficial owner of some or all of the shares owned by MGC 2013 LP, MGC 2018 LP and MGC QP 2018 LP and, therefore, a "ten percent holder" hereunder. ( 5 )Each of Joseph C. Cook, Jr., Joseph C. Cook III, Steven D. Singleton and Byron W. Smith are managing directors of MGC 2013 GP and share voting and investment power over the shares held by MGC 2013 LP. Each of Joseph C. Cook, Jr., Joseph C. Cook III, Steven D. Singleton, Byron W. Smith, Jay Ferguson and Rob Readnour are managing partners of MGC 2018 GP and share voting and investment power over the shares held by MGC 2018 LP and MGC QP 2018 LP. Each of the Reporting Persons disclaims beneficial ownership of the shares held by any other Reporting Persons reported herein, except to the extent of its pecuniary interest therein, and this report shall not be deemed an admission that the Reporting Persons are the beneficial owners of such securities for purposes of Section 16 or for any other purposes. ( 6 )The principal amount of the Convertible Promissory Note held by the designated Reporting Person was $1,200,000.00. Upon closing of the IPO, all unpaid principal and accrued interest outstanding on the Convertible Promissory Note (the "Conversion Amount") automatically converted into the number of shares of Common Stock equal to the Conversion Amount as of the date immediately prior to the closing date divided by 80% of the IPO price per share of $16.00 per share. ( 7 )Represents a purchase from the underwriters in the Issuer's IPO. ( 8 )Held directly by MGC 2018 LP. ( 9 )Held directly by MGC QP 2018 LP.

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).

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