PDCD1 and melanoma: Monoclonal antibodies targeting cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4), programmed cell death protein-1 (PD-1), and PD-ligand 1 (PD-L1), which are defined as immune checkpoint inhibitors (ICIs), have significantly improved the outcome of metastatic melanoma patients and so have become a pillar of advanced melanoma treatment [1,3].