CTLA4 and cancer: Cytotoxic T lymphocyte-associate antigen-4 (CTLA-4) is a critical negative regulator of immune responses49, 50, and its blockade by antibodies (for example, anti-CTLA-4) to inhibit the activities of immune-suppressive Tregs has been approved by FDA as a cancer immunotherapy approach currently used in the clinic51, 52, 53, 54.