PDCD1 and metastatic melanoma: Pembrolizumab, the first PD-1 checkpoint inhibitor, was approved in September 2014 for the treatment of patients with metastatic melanoma and shortly after, Nivolumab, another PD-1 checkpoint inhibitor, was approved in December 2014 and is currently used in treatment of many malignancies including melanoma, lung cancer, kidney cancer, and bladder cancer [6].