X0306 4 2020-05-05 0 0001278021 MARKETAXESS HOLDINGS INC MKTX 0001296969 Casper Stephen P C/O MARKETAXESS HOLDINGS INC. 55 HUDSON YARDS, 15TH FLOOR NEW YORK NY 10001 1 0 0 0 Common Stock, par value $0.003 per share 2020-05-05 4 S 0 200 476.53 D 13367 D Common Stock, par value $0.003 per share 2020-05-05 4 S 0 200 477.84 D 13167 D Common Stock, par value $0.003 per share 2020-05-05 4 S 0 1400 479.86 D 11767 D Common Stock, par value $0.003 per share 2020-05-05 4 S 0 200 480.98 D 11567 D Common Stock, par value $0.003 per share 42808 I By family trust The transaction was executed in multiple trades at prices ranging from $479.26 to $480.00. 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 Stephen P. Casper 2020-05-07