Studies of another IEI associated with T1DM, IPEX syndrome have shown that the responsible gene, FOXP3 (21, 22), plays a critical role in the function of CD4+CD25+ regulatory T (Treg) cells (23, 24), and its defects cause loss of peripheral Tregs, leading to autoreactive T cell activation and proliferation (20, 23, 24). The gene discussed is FOXP3; the disease is type 1 diabetes mellitus.