FOXP3 and immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome: With Tregs, XCI acts as a form of quality control of the FOXP3 gene; for example in the rare disease linked to the dysfunction of FOXP3, immunodysregulation polyendocrinopathy enteropathy X-linked (or IPEX) syndrome, IPEX females are heterozygous for the mutant FOXP3 and it is silent [25].