Immune checkpoint inhibitors (ICIs), including cytotoxic T-cell lymphocyte-4 (CTLA-4) and programmed death-1 (PD-1)/ligand-1 (PD-L1) inhibitors, are effective in the treatment of a rapidly increasing number of advanced cancer types, including malignant melanoma, non-small cell lung cancer, head and neck cancers, and genitourinary and hematologic malignancies [1–9]. This evidence concerns the gene CTLA4 and cancer.