X0306 4 2020-05-12 0 0001447362 CASTLE BIOSCIENCES INC CSTL 0001783292 MGC Venture Partners 2013 GP, LLC C/O MOUNTAIN GROUP CAPITAL, LLC 3835 CLEGHORN AVE., SUITE 300 NASHVILLE TN 37215 0 0 1 0 0001572764 MGC Venture Partners 2013, L.P. C/O MOUNTAIN GROUP CAPITAL, LLC 3835 CLEGHORN AVE., SUITE 300 NASHVILLE TN 37215 0 0 1 0 0001783291 MGC Venture Partners 2018 GP, LLC C/O MOUNTAIN GROUP CAPITAL, LLC 3835 CLEGHORN AVE., SUITE 300 NASHVILLE TN 37215 0 0 1 0 0001745133 MGC VENTURE PARTNERS 2018, L.P. C/O MOUNTAIN GROUP CAPITAL, LLC 3835 CLEGHORN AVE., SUITE 300 NASHVILLE TN 37215 0 0 1 0 0001749814 MGC VENTURE PARTNERS QP 2018, L.P. C/O MOUNTAIN GROUP CAPITAL, LLC 3835 CLEGHORN AVE., STE. 300 NASHVILLE TN 37215 0 0 1 0 Common Stock 2020-05-12 4 S 0 5187 36.61 D 1491668 I See footnotes Common Stock 2020-05-12 4 S 0 4601 37.46 D 1487067 I See footnotes Common Stock 2020-05-13 4 S 0 1131 36.08 D 1485936 I See footnotes Common Stock 115300 I See footnotes Common Stock 134700 I See footnotes The transactions on this Form 4 were made pursuant to a Rule 10b5-1 plan adopted by MGC Venture Partners 2013, L.P. ("MGC 2013 LP") on March 13, 2020. This transaction was executed in multiple trades at prices ranging from $36.00 to $36.96, inclusive. The price reported above reflects the weighted-average sale price. MGC 2013 LP hereby undertakes to provide upon request to the Securities and Exchange Commission staff, the issuer or a security holder of the issuer full information regarding the number of shares and prices at which the transaction was effected. Held directly by MGC 2013 LP. 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. 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. This transaction was executed in multiple trades at prices ranging from $37.00 to $37.98, inclusive. The price reported above reflects the weighted-average sale price. MGC 2013 LP hereby undertakes to provide upon request to the Securities and Exchange Commission staff, the issuer or a security holder of the issuer full information regarding the number of shares and prices at which the transaction was effected. This transaction was executed in multiple trades at prices ranging from $36.00 to $36.45, inclusive. The price reported above reflects the weighted-average sale price. MGC 2013 LP hereby undertakes to provide upon request to the Securities and Exchange Commission staff, the issuer or a security holder of the issuer full information regarding the number of shares and prices at which the transaction was effected. Held directly by MGC 2018 LP. Held directly by MGC QP 2018 LP. /s/ Joseph C. Cook III, Managing Member 2013 GP, LLC, the General Partner of MGC Venture Partners 2013, L.P. 2020-05-14 /s/ Joseph C. Cook III, Managing Member of MGC Venture Partners 2013 GP, LLC 2020-05-14 /s/ Jason S. Ferguson, Managing Partner of MGC Venture Partners 2018 GP, LLC, the General Partner of MGC Venture Partners 2018, L.P. 2020-05-14 /s/ Jason S. Ferguson, Managing Partner of MGC Venture Partners 2018 GP, LLC, the General Partner of MGC Venture Partners QP 2018, L.P. 2020-05-14 /s/ Jason S. Ferguson, Managing Partner of MGC Venture Partners 2018 GP, LLC 2020-05-14