F5 and hyperinsulinemic hypoglycemia, familial, 4: One possible site BS1 for FVa binding to FXa is the Arg334 region (legacy Arg306) in the A2 domain that involves either residues 339 to 353 in FV (legacy 311–325)35or a larger section in residues 335 to 376 (legacy 307–348).36In our dataset of 199 unique FV deficiency variants, mutations were found in 14% of the residues in the larger proposed binding region.