Immune checkpoints such as cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), programed cell death protein 1 (PD-1), and programed cell death 1 ligand 1 (PD-L1) act to negatively regulate T-cell–mediated immune responses that play a critical role in allowing cancer cells to evade the immune destruction. The gene discussed is PDCD1; the disease is cancer.