FOXP3 (a 431 amino acid protein) 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, at 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.