Susceptibility to insulin-dependent diabetes mellitus (IDDM) is determined by both genetic and environmental factors. The major genetic susceptibility to IDDM is conferred by genes in the HLA class II region on chromosome 6. In Caucasians, this susceptibility is thought to be determined by DQA1 and DQB1 genes including the presence of arginine at position 52 of the DQ alpha chain and the absence of aspartic acid at position 57 of the DQ beta chain. In Japanese subjects, IDDM has been found to be associated DR, DQA1, and DQB1 genes. Positive associations with arginine at position 52 of the DQ alpha chain have been demonstrated in Japanese IDDM patients. However, the majority of Japanese IDDM patients do not show positive association to the absence of aspartic acid at position 57 of the DQ beta chain. Susceptibility to IDDM in Japanese subjects appears to be due to combination of DR genes and DQA1-DQB1 haplotypes.