CTLA4 and autoimmune disease: Foxp3 is a master regulator of regulatory T cells (Tregs) and its mutations result in fatal autoimmune disease in mice and human.1–5 Among many known functions, Foxp3 is a transcriptional factor for expression of CTLA-4, which is constitutively expressed in Tregs.6,7Ctla4 deletion in mice phenocopies that of Foxp3.