PDCD1 and cancer: Understanding this complex biology has allowed NK cell cytotoxicity to be exploited in cancer therapies, like utilizing cytokine therapies, chimeric antigen receptor (CAR)-NK cell therapy, immune checkpoint inhibitors (ICIs) for CTLA-4 (cytotoxic T-lymphocyte-associated protein-4), PD-1 (CD279) and PD-L1 (CD274), and the use of inhibitors of inhibitory NK cell receptor-ligand interactions.