CTLA4 and cancer: Principles of immune checkpoint blockade in cancer therapy grew from early mouse models showing antibodies against T-cell and tumor co-inhibitor signals cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), programmed cell death protein 1 (PD-1) or programmed cell death ligand 1 (PD-L1) could induce tumor regression (3–6).