Not only is Foxp3 a useful Treg marker (which allows differentiation from activated CD4+ Teff cells), but its stable expression is required for Treg differentiation and function [12], as Foxp3 deficiency leads to a severe autoimmune-mediated multiorgan inflammation in mice [1,2] and to the related IPEX syndrome (immune dysregulation, polyendocrinopathy, enteropathy, X-linked) in humans [2,13-16]. Here, FOXP3 is linked to immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome.