PTPN22 and type 1 diabetes mellitus: That is, HLA-DQB1 in antigen presentation, CTLA4 in regulation of the expansion of T cell subsets, and PTPN22 in T cell receptor signaling, all contributing to immunological aberrations and progression to clinical disease, as in rheumatoid arthritis, systemic lupus erythematosus, and type 1 diabetes.