PDCD1 and melanoma: Currently, the treatment of melanoma is mainly divided into two therapeutic mechanisms [3]: (1) targeted therapy: small-molecule inhibitors targeting the mitogen-activated protein kinase (MAPK) signaling pathway; (2) immunotherapy: biological monoclonal antibodies block cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) and programmed cell death protein 1 (PD-1).