PDCD1 and melanoma: In the last few years, the discovery of the role of immune checkpoint molecules, e.g., cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), programmed cell-death protein 1 (PD1), and PD-ligand 1 (PD-L1), has enabled the detection of immune checkpoint inhibitors with promising results for melanoma treatment [67].