Immune checkpoints, including cytotoxic T-lymphocyte-associated antigen 4 (CTLA)-4, the programmed cell death protein 1 (PD-1) receptor, and its ligands (PD-L1 and PD-L2), suppress T-lymphocyte activity, which can detect cancer antigens as foreign and destroy them [150]. The gene discussed is PDCD1; the disease is cancer.