The IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked) syndrome due to FOXP3 gene mutations is unequivocal evidence that deficiency or dysfunction of nTreg cells can be a primary cause of not only autoimmune diseases such as type 1 diabetes, but also inflammatory bowel disease and allergy in humans. This evidence concerns the gene FOXP3 and type 1 diabetes mellitus.