PDCD1 and melanoma: ICIs were approved as early as 2011 to treat unresectable advanced melanoma after conventional therapy [13], and, as of September 2022, the US Food and Drug Administration (FDA) has approved nine drugs targeting four immune checkpoints, including cytotoxic T-lymphocyte-associated protein-4 (CTLA-4), programmed cell death-1 (PD-1), programmed death ligand-1 (PD-L1), and lymphocyte activation gene-3 (LAG-3).