X0306 4 2019-08-08 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 2019-08-08 4 S 0 500 345.14 D 53064 D Common Stock, par value $0.003 per share 2019-08-08 4 S 0 500 346.09 D 52564 D Common Stock, par value $0.003 per share 2019-08-08 4 S 0 300 347.10 D 52264 D Common Stock, par value $0.003 per share 2019-08-08 4 S 0 1120 348.46 D 51144 D Common Stock, par value $0.003 per share 2019-08-08 4 S 0 330 349.36 D 50814 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 $344.78 to $345.60. 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 $345.90 to $346.59. 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 $346.91 to $347.40. 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 $348.08 to $349.02. 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 $349.29 to $349.53. 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 2019-08-09