FOXP3 is a member of the forkhead/winged-helix family of transcriptional regulators and is highly conserved across mammals. FOXP3 is essential for normal immune homeostasis. FOXP3 is stably and constitutively expressed at a high level in CD25 + CD4 positive regulatory T cells, a low level in CD4-positive/CD25-negative cells, and is absent in CD4-negative/CD8-positive T cells. FOXP3 may be a master regulatory gene and a more specific marker of regulatory T cells than other T cells.
Defects in FOXP3 are the cause of immunodeficiency polyendocrinopathy, enteropathy, X-linked syndrome (IPEX) [MIM:304790]; also known as X-linked autoimmunity-immunodeficiency syndrome. IPEX is characterized by neonatal onset insulin-dependent diabetes mellitus, infections, secretory diarrhea, trombocytopenia, anemia and eczema. It is usually lethal in infancy.