Defective development or depletion of Treg in animals, such as the naturally arising Foxp3+CD4+CD25+ Treg (nTreg), results in the development of autoimmune diseases, including type 1 diabetes (T1D) [1], [2], [3], [4], [5], [6], [7]. The gene discussed is CD4; the disease is type 1 diabetes mellitus.