The autoimmune disease type 1 diabetes mellitus (insulin-dependent diabetes mellitus, IDDM) has a multifactorial etiology. So far, the major histocompatibility complex (MHC) is the only major susceptibility locus that has been identified for this disease and its animal models. The Komeda diabetes-prone (KDP) rat is a spontaneous animal model of human type 1 diabetes in which the major susceptibility locus Iddm/kdp1 accounts, in combination with MHC, for most of the genetic predisposition to diabetes. Here we report the positional cloning of Iddm/kdp1 and identify a nonsense mutation in Cblb, a member of the Cbl/Sli family of ubiquitin-protein ligases. Lymphocytes of the KDP rat infiltrate into pancreatic islets and several tissues including thyroid gland and kidney, indicating autoimmunity. Similar findings in Cblb-deficient mice are caused by enhanced T-cell activation. Transgenic complementation with wildtype Cblb significantly suppresses development of the KDP phenotype. Thus, Cblb functions as a negative regulator of autoimmunity and Cblb is a major susceptibility gene for type 1 diabetes in the rat. Impairment of the Cblb signaling pathway may contribute to human autoimmune diseases, including type 1 diabetes.