The master lineage-specific transcription factor FOXP3 phenotypically defines the Treg lineage, and genetic deficiency of FOXP3 confers a lack of Tregs and a fatal autoimmune disease known as immunodysregulation polyendocrinopathy enteropathy X-linked in humans and scurfy in mice [9, 10]. The gene discussed is FOXP3; the disease is autoimmune disease.