The prototype for this group is the syndrome of Immune Dysregulation, Polyendocrinopathy, Enteropathy, X-linked (IPEX) (29) caused by mutations in the Forkhead Box P3 (FOXP3) gene that results in the defective development of CD4+CD25+ regulatory T cells (Tregs). The gene discussed is FOXP3; the disease is immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome.