The AP1 epitope lies within the region spanning R218 to Y228. (A) Alignment of the human and canine sequences between Cys211 and Tyr228. Conserved amino acids are boxed and the locations of the α1 helix and β13 strands are noted. This designation is from the structure published by Uff et al,⁴  where they designated the β13 strand as extending from residue 226 to residue 228. In the 2 structures of the GP Ibα N-terminus in complex with the VWF A1 domain,^3,35  the 13 β strand extends from residue 227 to residue 232, although it is not formally designated as β13. (B) Binding of the GP Ibα monoclonal antibody AP1 to CHO cells expressing chimeric GP Ibα was assessed by flow cytometry. Left panels show nonconserved residues of wild-type human GP Ibα within the sequence depicted in panel A mutated to corresponding canine residues; right panels, nonconserved canine residues mutated to the corresponding human residues using C1-225 as the template.