PDCD1 and cancer: Immune checkpoint treatment, such as immunotherapy strategies aiming at the programmed cell death ligand 1 (PD-L1)/programmed cell death 1 (PD-1) pathway has achieved significant success, but because of the heterogeneity of tumor and the individual immune system, PD-L1/PD-1 antagonists still show a low response rate in many patients for controlling malignant tumors.