CTLA4 and cancer: PD-1 is a negative regulatory receptor expressed on the surface of activated T cells and B cells; thus it acts as an immune checkpoint.[4] Inhibitory effects of nivolumab on immune checkpoints enhance the antineoplastic immune response.[5] Cytotoxic T-lymphocyte-associated protein-4 (CTLA-4) is another immune checkpoint; additionally, human anti-CTLA-4 IgG1 antibody, ipilimumab, has antineoplastic activity.[5] These immune checkpoint inhibitors have been incorporated into many cancer treatment regimens.