PDCD1 and metastatic melanoma: The blockade of cytotoxic T lymphocyte antigen-4 (CTLA-4), programmed cell death-1 (PD-1), or its ligand PD-L1 with monoclonal antibodies (e.g., ipilimumab, pembrolizumab, or nivolumab) considerably prolongs the survival of patients with advanced or metastatic melanoma (1).