PDCD1 and metastatic melanoma: Since the first FDA approval of anti-cytotoxic T lymphocyte antigen 4 (CTLA-4) for treatment of metastatic melanoma, a range of anti-programmed cell death protein 1 (PD-1) and anti-PD1 ligand 1 (PD-L1) therapies have been FDA approved for a large number of cancer types, including melanoma, non-small cell lung cancer (NSCLC), kidney cancer, and liver cancer [3].