PDCD1 and metastatic melanoma: Anti-programmed cell death-1 (anti-PD-1) antibodies such as nivolumab and pembrolizumab and anti- cytotoxic T lymphocyte-associated antigen 4 (anti-CTLA-4) antibodies such as ipilimumab clearly exhibited a benefit in terms of progression free survival (PFS) and overall survival (OS) in patients with metastatic melanoma [5].