PDCD1 and cancer: Immunotherapy, including approved immune checkpoint inhibitors (ICIs), anti-cytotoxic T-lymphocyte-associated protein 4 (anti-CTLA-4), anti-programmed cell death-1/programmed cell death-Ligand 1 (anti-PD-1/PD-L1) antibody and chimeric antigen receptor (CAR) T-cell therapy, has achieved considerably improved clinical outcomes in cancer treatment [1,2].