X0306 4 2021-06-14 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 2021-06-14 4 M 0 7000 103.30 A 20587 D Common Stock, par value $0.003 per share 2021-06-14 4 S 0 4713 448.40 D 15874 D Common Stock, par value $0.003 per share 2021-06-14 4 S 0 2287 449.13 D 13587 D Stock Option (right to buy) 103.30 2021-06-14 4 M 0 7000 0.00 D 2022-01-31 Common Stock 7000 6433 D The transactions were executed in multiple trades at prices ranging from $447.89 to $448.86. 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 $448.97 to $449.52. 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 2021-06-15