The four models are similar to the previously reported B21 haplotype. The peptide-binding domain is formed by two helices at the top and an eight-stranded sheet at the bottom (Fig. 4A) . The pairwise root mean-square differences (RMSD) between the Ca positions of 3BEV and the structures of the B4, B12, B15, and B19 haplotypes are very small (0.23, 0.20, 0.24, and 0.30 respectively). However, there are some variations around the peptide-binding grooves and these differences may determine differences in their peptide-binding properties. Specific residues in the binding groove have an essential role in peptide binding. Figure 4B shows the key residues comprising the binding site, when all the models and the template were superimposed.