PDCD1 and melanoma: Since the cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) antibody ipilimumab has become the first ICI used in melanoma therapy, anti-PD-L1/PD-1 agents have been approved for the treatment of melanoma, non-small-cell lung cancer, head and neck squamous cell carcinoma, MMS-H/dMMR solid tumors, Hodgkin’s lymphoma, and other cancers (13–17).