The 1.31-Mb Idd18.2 locus contains 18 protein-encoding genes, including two strong candidates based on their known immune functions, Ptpn22 and Nras. The human ortholog of Ptpn22, PTPN22, is associated with T1D and 15 other autoimmune diseases, including rheumatoid arthritis, Graves disease, Addison disease, and systemic lupus erythematosus (11). Here, PTPN22 is linked to type 1 diabetes mellitus.