PDCD1 and cancer: Monoclonal antibodies (mAbs) directed against the programmed cell death protein-1 (PD-1) and against the cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), are exhibiting promising cancer treatment effects, enhance antitumor immunity and improve patient survival, therefore have been approved for the therapies of non-small cell lung cancer and melanoma in clinics [4–6].