HLA-C and type 1 diabetes mellitus: Although NCAAH is not autoimmune, the location of <i>CYP21A2</i> within the major histocompatibility complex (MHC)--a genomic region rich in immunoregulatory genes--may contribute to a shared genetic susceptibility background in a subset of patients.The coexistence of NCCAH and T1DM is uncommon but may be partially explained by the location of the <i>CYP21A2</i> gene within the major histocompatibility complex (MHC), a genomic region associated with autoimmune susceptibility.