In type I diabetes, a T cell-mediated, HLA-DQ2 (DQA1*05:01-DQB1*02:01) and -DQ8 (DQA1*03:01-DQB1*03:02)-associated autoimmune disease directed at pancreatic β cells, insulin B-chain9-23 (B:9-23) is a key epitope presented by MHC-class II to CD4+ T cells targeting pancreatic β-cells. Here, CD4 is linked to type 1 diabetes mellitus.