PDCD1 and melanoma: The development of various immunotherapeutic strategies, especially with regard to so called ‘immune checkpoints’, targeting the cytotoxic-T-lymphocyte-associated protein-4 (CTLA-4) or the programmed cell death 1 (PD1) receptor and its ligand PD-L1, have revolutionized the field of melanoma therapy in recent years [10,11].