X0306 4 2020-10-29 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 2020-10-29 4 M 0 13432 103.30 A 43624 D Common Stock, par value $0.003 per share 2020-10-29 4 M 0 3830 156.85 A 47454 D Common Stock, par value $0.003 per share 2020-10-29 4 S 0 200 551.27 D 47254 D Common Stock, par value $0.003 per share 2020-10-29 4 S 0 1440 552.06 D 45814 D Common Stock, par value $0.003 per share 2020-10-29 4 S 0 9324 553.21 D 36490 D Common Stock, par value $0.003 per share 2020-10-29 4 S 0 100 554.23 D 36390 D Common Stock, par value $0.003 per share 2020-10-29 4 S 0 600 556.69 D 35790 D Common Stock, par value $0.003 per share 2020-10-29 4 S 0 5598 557.81 D 30192 D Stock Option (right to buy) 103.30 2020-10-29 4 M 0 13432 0.00 D 2022-01-31 Common Stock 13432 13433 D Stock Option (right to buy) 156.85 2020-10-29 4 M 0 3830 0.00 D 2023-01-15 Common Stock 3830 0 D The transactions were executed in multiple trades at prices ranging from $550.79 to $551.74. 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 $551.82 to $552.61. 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 $553.00 to $553.99. 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 $556.50 to $556.93. 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 stock options were fully exercisable. /s/ Jason Edelstein, as Attorney-in-Fact for Nicholas Themelis 2020-10-30