PDCD1 and neoplasm: Programmed cell death protein 1 (PD-1) bound to programmed death ligand 1 (PD-L1) can inhibit the activation of cytotoxic T cells and promote tumor cell evasion of the immune system; immune checkpoint inhibitors (ICIs) block the binding of PD-1 to PD-L1 and thereby enhance the body’s antitumor activity.