PDCD1 and metastatic melanoma: Until now, systemic therapy for metastatic melanoma has been ineffective, but recent successes in the development of new therapies including mitogen-activated protein kinase (MAPK) pathway inhibitors, anti-Cytotoxic T-Lymphocyte Antigen-4 (CTLA-4) and Programmed cell death protein 1 (PD-1)/Programmed cell death 1 ligand 1 (PD-L1) 'blocking antibodies have all yielded promising results, enlarging the variety of therapeutic options for patients [1, 2].