FOXP3 and immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome: These FOXP3+ CD25high Tregs are essential for maintenance of self-tolerance because loss-of-function mutations in Foxp3 cause an early-onset, lethal autoimmune disorder known as immunodysregulation polyendocrinopathy enteropathy X-linked syndrome (IPEX) in humans and scurfy in mice (5, 6).