PDCD1 and melanoma: The introduction of ICIs targeting key immune checkpoints such as cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), programmed cell death protein 1 (PD-1) on T cells, its ligand on tumor cells (PD-L1), or recently the lymphocyte activation gene-3 (LAG-3) have become integral components in the treatment of malignant melanoma.