CTLA4 and cancer: Tumors use immune checkpoint inhibitory pathways, mediated by cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), PD-1 (programmed death 1), inducible T-cell costimulatory (ICOS), lymphocyte-activation gene 3 (LAG-3), T-cell immunoglobulin, and mucin-domain containing-3 (TIM-3), to prevent T lymphocyte proliferation and activation of natural killer (NK) cells, which are responsible for cancer cell lysis.