PDCD1 and melanoma: Recent advances in the understanding of melanoma biology and the success of immunotherapy, such as anti-Cytotoxic T-Lymphocyte Antigen 4 (CLTA-4) and anti-Programmed cell death protein 1 (PD-1) antibodies, and therapies targeting serine/threonine kinase (BRAF) and mitogen-activated protein kinase 1 (MEK) have led to significant prognostic improvements in unresectable stage III and metastatic stage IV CM patients [12,13,14].