PDCD1 and cancer: Monoclonal antibodies targeting CTLA-4 (ipilimumab), PD-1 (e.g., pembro-lizumab and nivolumab), and PD-L1 (e.g., avelumab, atezolizumab, and durvalumab) have been approved by the Food and Drug Administration of USA (FDA) in the treatment of various malignant tumors, such as melanoma, non-small cell lung cancer, renal cell carcinoma, urothelial carcinoma, and lymphoma [9].