X0306 4 2021-09-07 0 0001278021 MARKETAXESS HOLDINGS INC MKTX 0001296468 Themelis Nicholas C/O MARKETAXESS HOLDINGS INC. 55 HUDSON YARDS, 15TH FLOOR NEW YORK NY 10001 0 1 0 0 Chief Information Officer Common Stock, par value $0.003 per share 2021-09-07 4 S 0 100 456.85 D 13487 D Common Stock, par value $0.003 per share 2021-09-07 4 S 0 800 458.58 D 12687 D Common Stock, par value $0.003 per share 2021-09-07 4 S 0 500 460.36 D 12187 D Common Stock, par value $0.003 per share 2021-09-07 4 S 0 100 461.31 D 12087 D Common Stock, par value $0.003 per share 2021-09-07 4 S 0 100 463.33 D 11987 D Common Stock, par value $0.003 per share 2021-09-07 4 S 0 200 466.01 D 11787 D Common Stock, par value $0.003 per share 2021-09-07 4 S 0 200 473.40 D 11587 D This transaction was effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person. The transactions were executed in multiple trades at prices ranging from $458.00 to $458.93. The price above reflects the weighted average sales price. Detailed information regarding the number of shares sold at each separate price will be provided upon request by the Commission staff, the Issuer or a security holder of the Issuer. The transactions were executed in multiple trades at prices ranging from $460.00 to $460.82. The price above reflects the weighted average sales price. Detailed information regarding the number of shares sold at each separate price will be provided upon request by the Commission staff, the Issuer or a security holder of the Issuer. /s/ Jason Edelstein, as Attorney-in-Fact for Nicholas Themelis 2021-09-09