CTLA4 and type 1 diabetes mellitus: The BB rat type 1 diabetes susceptibility locus Ian4/Iddm1 [7] affects T lymphocyte development whereas the Cblb (KDP rat) [8] and CTLA4 [9] (in humans and NOD mice) susceptibility genes highlight the importance of the regulation of T cell activation, expansion and homeostasis in the periphery, and perhaps in the thymus as well.