PDCD1 and cancer: Immune therapy is a new category of cancer therapy, which is aimed to reactivate host antitumor immunity either by interrupting the immune checkpoint complexes hijacked by tumor cells [e.g., blocking antibodies for programmed cell death protein 1/programmed death ligand 1 (PD-1/PD-L1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4)] or by infusing genetically engineered immune cells [e.g., chimeric antigen receptor (CAR)-T-cell therapy].