These agents restore the immune system capacity to fight cancer through blocking the action of major immune checkpoint proteins that suppress immune responses such as programmed cell death protein 1 (PD-1), programmed death-ligand 1 (PD-L1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), which tumors exploit for immune escape. The gene discussed is PDCD1; the disease is cancer.