Comparison of the amino acid sequences of human and murine factor XI. The amino acid numbering system used is for human factor XI.17 Amino acids −18 to −1 represent the leader peptide and the N-terminal heavy chain is numbered 1-369. The C-terminal catalytic light chain is numbered 1-238 and begins immediately after the heavy chain sequence. Alignment required the insertion of one gap in the human factor XI sequence after amino acid 325 of the heavy chain and one gap in the murine sequence after amino acid 20 of the light chain. The black arrowhead after Arg³⁶⁹ of the heavy chain designates the factor XIIa and thrombin activation cleavage site. The positions of the serine protease catalytic triad of His, Asp, Ser in the light chain are designated by black circles. The asterisk (*) designates the cystine residue at position 321 of the heavy chain involved in the disulfide bond connecting the two polypeptides of the homodimer. Amino acid positions with identical residues are enclosed in the shaded boxes.