FOXP3 and immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome: Treg cells expressing the transcription factor forkhead box P3 (FoxP3) have been studied extensively and deficiency of Tregs causes a severe and lethal autoimmune syndrome in humans (termed IPEX, immunodysregulation polyendocrinopathy enteropathy X-linked syndrome) and mice [125, 126].