PDCD1 and cancer: Recently, several monoclonal antibodies against cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) (e.g., ipilimumab), programmed cell death protein 1(PD-1) (e.g., nivolumab and pembrolizumab), and programmed cell death protein ligand 1(PD-L1) (e.g., atezolizumab and durvalumab) have been introduced to treat many different types of cancers [3–5].