X0508 4 2023-08-14 0 0001278021 MARKETAXESS HOLDINGS INC MKTX 0001251713 CONCANNON CHRISTOPHER R C/O MARKETAXESS HOLDINGS INC. 55 HUDSON YARDS, 15TH FLOOR NEW YORK NY 10001 1 1 0 0 Chief Executive Officer 0 Common Stock, par value $0.003 per share 2023-08-14 4 P 0 104 235.10 A 43000 D Common Stock, par value $0.003 per share 2023-08-14 4 P 0 404 236.49 A 43404 D Common Stock, par value $0.003 per share 2023-08-14 4 P 0 2203 238.05 A 45607 D Common Stock, par value $0.003 per share 2023-08-14 4 P 0 900 239.26 A 46507 D Common Stock, par value $0.003 per share 2023-08-14 4 P 0 600 240.16 A 47107 D Common Stock, par value $0.003 per share 2023-08-14 4 P 0 59 240.90 A 47166 D The transactions were executed in multiple trades at prices ranging from $234.52 to $235.12. 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 $236.06 to $236.64. 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 $237.65 to $238.37. 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 $238.72 to $239.62. 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 $239.72 to $240.37. 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/ Scott Pintoff, as Attorney-in-Fact for Christopher R. Concannon 2023-08-15