Scurfy mice lacking the Treg-specific transcription factor forkhead box P3 (foxp3) develop a lymphoproliferative disorder (Brunkow et al., 2001) and humans with mutations in FOXP3 can develop IPEX (immunodysregulation, polyendocrinopathy, and enteropathy, X-linked; Bennett et al., 2001). Here, FOXP3 is linked to immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome.