PDCD1 and cancer: As a new drug class, immunology checkpoint inhibitor (ICI) therapies, such as those involving anti-CTLA4 (3), anti-programmed cell death protein 1 (PD-1) (4) and anti-programmed death-ligand 1 (PD-L1) (5), show promise in the clinical treatment of several cancer types, especially melanoma and lung cancer (6), and they cause cancer to become a chronic disease by activating tumor immune cells in the tumor microenvironment (TME).