CD4 and autoimmune disease: Regulatory T cells (Tregs), a subtype of CD4 T cells, control the activation status of many different immune cells including other (conventional) CD4 T cells to enforce peripheral tolerance.1 Breach of this check and balance machinery resulting from a deficit in Treg number or functionality has been clearly shown to contribute to the pathogenesis of autoimmune diseases, allergies, and chronic inflammatory diseases.2, 3 On the opposite end of the spectrum, unwarranted immune suppression by Tregs is among the causes of immune evasion in cancer4 and pathogenic diseases.