However, it is worth mentioning that the concept of Foxp3 as a “lineage-specifying factor” of Tregs is an over-simplification, as suggested by three lines of evidence: (i) Foxp3 is not sufficient in itself to determine the full Treg transcriptional profile (Hill et al., 2007); (ii) Foxp3 is expressed by (human) Teffs following activation, without imparting the phenotype associated with Tregs; (iii) humans with IPEX syndrome have heterogeneous T cell abnormalities, including dysfunction in Teffs (Bacchetta et al., 2006). Here, FOXP3 is linked to immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome.