IKZF1 and immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome: ,12 The prototype IEI due to loss of Tregs is the immunodysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX), which is caused by biallelic loss-of-function mutations in FOXP3. The spectrum of IEI characterized by immune dysregulation associated with Treg defects includes 10 genes, according to the current IUIS classification (FOXP3, CTLA4, LRBA, DEF6, IL2RA, IL2RB, STAT3, BACH2, FERMT1 and IKZF1),5 though additional genes, such as STAT5b, PIK3CD and DOCK8 have been associated with Treg dysfunction.11