PDCD1 and melanoma: Numerous clinical studies in metastatic cancer, including malignant melanoma, demonstrate high efficacy and manageable toxicity by using FDA-approved immune checkpoint inhibitors against the cytotoxic T lymphocyte antigen-4 (CTLA-4) and/or programmed cell death 1 (PD-1)/PD-1 ligand (PD-L1) axis; thus, immunotherapy has rapidly become a standard treatment modality in oncology.