PDCD1 and metastatic melanoma: ICB, including programmed cell death protein 1 (PD-1), programmed cell death ligand 1 (PD-L1), and cytotoxic T lymphocyte antigen 4 (CTLA-4) monoclonal antibodies, have shown antitumor efficacies in multiple advanced solid tumors since the initial approval of CTLA-4 inhibitors for metastatic melanoma in 2011 by the US Food and Drug Administration (FDA) (2).