FOXP3 and immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome: Foxp3 has been shown to be the key regulator of Treg cell differentiation and function, as demonstrated by the devastating effects of loss-of-function mutations in the Foxp3 gene, leading to the lethal IPEX syndrome (immune dysregulation, polyendocrinopathy, enteropathy, X-linked).