PDCD1 and metastatic melanoma: The use of checkpoint inhibitor immunotherapy (CII) through immune checkpoint inhibitors (ICIs), such as ipilimumab (cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) target) and nivolumab and pembrolizumab (inhibitors of programmed cell death protein-1 (PD-1)), have revolutionized the treatment of metastatic melanoma.