Maximum likelihood tree for nucleotide sequences (690 bp) in the non-peptide-binding region (PBR) of MHC DRB alleles. The sequence data of MHC DRB alleles, including those of humans, chimpanzees, and macaques, were obtained from IMGT/HLA and IPD databases. HLA-DRB1 alleles are indicated in bold. Arrow indicates the Patr-DRB1*03 lineage, which is a sister group of HLA Group A alleles. Only bootstrap values > 80% are shown. Two HLA-DQB1 sequences were used as an outgroup. The evolutionary distances were computed using the Hasegawa-Kishino-Yano (HKY) model. HLA Group A and HLA Group B indicate two major phylogenetic groups of HLA-DRB1 alleles. HLA, humans; Patr, chimpanzees; Mamu, rhesus monkeys; Mafa, crab-eating macaques.