FOXP3 and immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome: For example, studies of immunedysregulation-polyendocrinopathy-enteropathy-X-linked syndrome (IPEX), which is caused by FOXP3 gene mutations, have critically established the fundamental role of regulatory T cells (Tregs) in natural tolerance, and PID-AID associations have consolidated the notion of dominant tolerance.