In regard to Tregs, Forkhead box P3 (Foxp3) is exclusively expressed on Tregs and contributes to their suppressive function, as strongly evidenced in a severe, rare X-linked fatal autoimmune disease known as immune dysregulation polyendocrinopathy enteropathy, X-linked (IPEX) syndrome, where mutations of this transcription factor occur [35]. This evidence concerns the gene FOXP3 and autoimmune disease.