FOXP3 and immune system disorder: In support of the crucial roles played by Foxp3 in Treg fate determination and immune homeostasis, Foxp3 mutations have been recognized as responsible for human Immune Dysfunction, Polyendocrinopathy, Enteropathy, X-linked (IPEX) syndrome (6, 7), and for the phenotype of scurfy mutant mice (8), both characterized by fatal autoimmune lymphoproliferation linked to severe defects in Treg development/functions.