PDCD1 and cancer: The activation of inhibitory immune checkpoint proteins, such as cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), programmed cell death protein 1 (PD-1), and programmed death-ligand 1 (PD-L1), impairs the T cell ability to activate, recognize and eliminate cancer cells, allowing uncontrolled cancer growth by bypassing antitumor immune surveillance.