Schematic diagram of the factor XI A3 domain.

The primary amino acid sequence and disulfide bond structure of the A3 domain from human factor XI is shown.13 The positions of amino acids for putative binding sites for platelets (●) and for factor IX () are highlighted. ○ indicate that amino acids are required for both platelet and factor IX binding. The arrows indicate the positions of the Q226R and S248N amino acid substitutions.