HLA-DR51 is a HLA-DR serotype that recognizes the antigens encoded by the minor DR locus HLA-DRB5. DRB3, DRB4, and DRB5 are minor DR beta encoding loci, they have been recognized as having distinct evolution, having diverged from DRB1 approximately 4 million years ago. DRB5 locus is only apparent in a small subset of DR haplotypes, and most individuals lack DRB5.