Localization of the AP1 epitope to the C-terminal flank domain. (A) Disulfide loops carboxyl terminal to the GP Ibα leucine-rich repeats. The loops are formed by disulfide bonds between Cys209 and Cys248 and between Cys211 and Cys264. (B) Binding of monoclonal antibody AP1 to canine-human GP Ibα chimeras. C plus the number denotes the sequence of human GP Ibα replaced by the corresponding canine sequence. These studies narrow the epitope to residues 201 to 225 of human GP Ibα.