PDCD1 and metastatic melanoma: ICIs, which target programmed cell death-1 (PD-1), programmed cell death ligand-1 (PD-L1), and cytotoxic T-lymphocyte-associated protein-4 (CTLA-4), have extended survival in many tumor types, including metastatic melanoma that has historically had poor survival due to low response to traditional cytotoxic regimens.