Likewise, T1D is considered a hallmark (3) of the fatal multiorgan autoimmune syndrome (IPEX; immune dysfunction, polyendocrinopathy, enteropathy, X-linked) affecting humans with abrogated Treg cell function due to genetic FOXP3 gene mutations (4, 5), demonstrating that a severe Treg cell defect is sufficient to promote destructive β cell autoimmunity independently of other genetic and environmental factors. Here, FOXP3 is linked to type 1 diabetes mellitus.