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.