The exons sequenced during the process of donor selection for allogeneic HCT encode critical portions of the peptide-binding grooves of class I and class II MHC molecules. (A) Structure of the extracellular portion of the HLA-A*02:01:01 molecule, indicating the regions encoded by exons 2 (blue), 3 (green), and 4 (red) of the A*02:01:01 allele of the HLA-A gene. (B) Structure of HLA-A*02:01:01 with the peptide VLHDDLLEA, the minor histocompatibility antigen HA-1, bound in its peptide-binding groove. (C) Structure of the extracellular portion of the heterodimeric HLA-DR4 molecule, showing the moieties encoded by exons 2 (light blue) and 3 (red) of the HLA-DRB1 gene, as well as the moiety encoded by exon 2 of the minimally polymorphic HLA-DRA gene (pale yellow). (D) Structure of HLA-DR4 with a peptide derived from influenza hemagglutinin, PKYVKQNTLKLAT, bound in its peptide-binding groove. The HLA-A*02:01:01 and HLA-DR4 structures were derived from Protein Data Bank (PDB) accession nos. 3D25107 and 1J8H,108 and rendered with Pymol.109
