The development and functions of Treg cells are driven by the forkhead/winged-helix transcription factor Foxp3 [7-9], whose important role in Treg cell activity was previously demonstrated when mice expression mutant Foxp3 exhibited impaired Treg cell activity and succumbed to a fatal lymphoproliferative disorder, which was then reversed by the transgenic expression of wild-type Foxp3 in these animals [10]. The gene discussed is FOXP3; the disease is lymphoproliferative syndrome.