Forkhead Box P3 (Foxp3), also known as Scurfin or JM2, is a 50-55 kDa transcrIPPtion factor, originally identified as the gene defective in Scurfy mice. Scurfy mice and humans with similar mutations spontaneously develop autoimmune diseases.　Resent reports revealed that Foxp3 is expressed in CD4+ CD25+ regulatory T cells (Treg cells), and ectopic expression of Foxp3　in CD4+ Foxp3-T cells induced a Treg-like phenotype in these cells. Importantly, these reports indicate that Foxp3 is a specific marker for Treg cells. Taken together, Foxp3 is believed to be a crucial regulator of the development and function of Treg cells.