In particular, treatments that block T cell “checkpoints”—such as the negative regulator proteins cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and programmed cell death 1 (PD-1)/programmed death ligand-1 (PD-L1)—unleash tumor-specific immune responses and are now FDA-approved to treat melanoma, lung cancer, and renal cell carcinoma. This evidence concerns the gene PDCD1 and melanoma.