PDCD1 and metastatic melanoma: Immune checkpoint inhibitors like the anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) antibody ipilimumab and the anti-programmed cell death protein 1 (PD-1) antibodies nivolumab and pembrolizumab have demonstrated remarkable efficacy in boosting T-cell responses against metastatic melanoma [8].