Immune checkpoint inhibitors (CPIs), such as the cytotoxic T lymphocyte associated antigen-4 (CTLA-4) antibody ipilimumab and the programmed cell death (PD-1)/programmed cell death ligand-1 (PD-L1) antibodies nivolumab, pembrolizumab, and atezolizumab, were the first drugs approved for use in immunotherapy to treat cancer (1–3). Here, CTLA4 is linked to cancer.