CD4 and immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome: Additionally, the rare disorder of the immune system, immune dysregulation, polyendocrinopathy, enteropathy X-linked (IPEX) syndrome, is caused by mutations in the FOXP3 gene that result in defective development of CD4+ CD25+ Treg; children with IPEX syndrome demonstrate high levels of serum IgE (reviewed in [37]).