Localization of factor VIII heavy chain-reactive antibodies. The A1 and A2 domains are separated by an acidic region (hatched). Cleavage sites for thrombin3 and APC^3,10 are designated by arrowheads. Antibody 58.12 was prepared against a 13 residue peptide composed of the heavy chain amino terminal sequence. The C5 epitope has been mapped to residues 351-365³⁵ and is retained as the C-terminus of the A1 subunit after thrombin cleavage. R8B12 recognizes an epitope near the C-terminus of the A2 subunit.10 Epitopes for the antipeptide polyclonal antibodies are as indicated.