CTLA4 and neoplasm: Checkpoint inhibitors are monoclonal antibodies that impede the activity of inhibitory molecules on the surface of tumor cells that typically reduce immune response; they take the “brakes” off the immune system so that tumor cells can be recognized.2,11 Clinical antibodies are available for three ligands: cytotoxic T-lymphocyte antigen 4 (CTLA-4), programmed death 1 (PD-1), and programmed death ligand 1 (PDL-1) (Figure 1).