Immune checkpoint inhibitors (ICIs), including those targeting programmed death-1 (PD-1)/programmed death-ligand 1 (PD-L1) and cytotoxic T-lymphocyte protein 4 (CTLA-4), have been used to improve survival in patients with metastatic melanoma, renal cell carcinoma, head and neck cancer, and NSCLC. This evidence concerns the gene CTLA4 and metastatic melanoma.