HLA-DRB1 and type 1 diabetes mellitus: Notably, DRA lacks polymorphisms in the peptide binding region and acts as the sole alpha chain for DRB1, DRB3, DRB4, and DRB5. The HLA-DR4 (DRB1*0405–DQB1*0401) and HLA-DR9 (DRB1*0901–DQB1*0303) haplotypes were primarily associated with T1D in East Asian populations (40).