Epitope mapping anti–factor V antibodies with recombinant factor V C2 domain chimeras. The complete domain structure of rHFV des B is shown at the top, and the expanded domain structures of the light chain portions of the individual chimeras are shown below. The white boxes indicate factor V–derived sequences, and the shaded boxes indicate factor VIII–derived sequences. Chimera 1A substituted amino acids 2282 through 2332 of factor VIII for amino acids 2149 through 2196 of factor V. Chimera 2A substituted amino acids 2223 through 2332 of factor VIII for amino acids 2088 through 2196 of factor V. Chimera 3A substituted amino acids 2173 through 2332 of factor VIII for amino acids 2037 through 2196 of factor V. Chimera 5A substituted amino acids 2173 through 2222 of factor VIII for amino acids 2037 through 2087 of factor V. Chimera 7A substituted amino acids 2223 through 2281 of factor VIII for amino acids 2088 through 2148 of factor V. The antibodies that immunoprecipitated the factor V C2 domain are listed on the right side. Immunoprecipitation of [³⁵S]methionine metabolically labeled mutants and analysis by SDS-PAGE was performed as described in Materials and Methods. +, the antibody immunoprecipitated the specific factor V mutant; −, it did not.