PDCD1 and cancer: In the background of immunotherapy gradually becoming one of the four pillars of tumor therapy, PD-1 and its immunotherapy ligands PD-L1 and PD-L2 have come into sight, which shows an encouraging effect in treating some cancers such as melanoma, non-small-cell lung cancer, renal cell carcinoma, Hodgkin’s lymphoma, bladder cancer and so on [4,15,16].