Figure 1.

Gene organization of HLA-DR haplotypes. DR haplotypes have varying numbers of genes, some of which encode a polypeptide chain (filled boxes) and others are pseudogenes that have no detectable product (open boxes). DR molecules are comprised of two polypeptide chains, an α chain and a β chain. All DR haplotypes have a DRA gene that encodes the relatively invariant α chain and a DRB1 gene that encodes the β chain of the DR1-DR18 antigens. Some haplotypes carry an additional gene, DRB3, 4, or 5, that encodes the β chain of the DR52, 53, or 51 molecules, respectively. DR haplotypes can be grouped into families defined by the number of DRB genes present as shown in the diagram.