CD4 and autoimmune disease: It builds on the knowledge that a lack of Treg function due to mutation of the Treg lineage-defining transcription factor FOXP3 leads to the X-linked autoimmune syndrome immune dysregulation, polyendocrinopathy, enteropathy X-linked syndrome (IPEX) (71), and, upon the seminal observation by Sakaguchi and colleagues, that giving CD4+CD25+ Treg could prevent autoimmune disease (2).