PDCD1 and cancer: The appearance of immune checkpoint inhibitors (ICI), including anti- cytotoxic T lymphocyte antigen-4 (anti-CTLA-4), and programmed cell death-1 (PD-1)/programmed death-ligand 1 (PD-L1) monoclonal antibodies, constitutes one of the major medical breakthroughs in oncology treatment, leading to impressive clinical results in different types of cancer (1).