Three-dimensional structure of the human GPIIIa PSI and EGF1 domains. (A) Note that the PSI domain lies between the hybrid and EGF1 domains of GPIIIa and that polymorphic amino acid 33, which controls expression of the HPA-1a (Pl^A1) epitope, is directly opposite the linearly distant but conformationally close EFG1 domain. (B) Mutation to alanine of Cys₄₃₅, which links the EGF1 domain to the PSI domain via a disulfide bond with Cys₁₃, has previously been shown to result in the loss of binding of some, but not all, maternal anti–HPA-1a alloantibodies, leading to speculation that nonpolymorphic amino acids in EGF1 constitute part of the epitope for these so-called type II antibodies.