Therapy with immune checkpoint inhibitors (ICI) that target the cytotoxic T-lymphocyte-associated protein-4 (CTLA-4; targeted by ipilimumab) and programmed cell death protein-1 (PD-1; targeted by pembrolizumab and nivolumab), either alone or in combination, achieves durable response rates in a variety of cancer types (1–3). The gene discussed is CTLA4; the disease is cancer.