CTLA4 and cancer: Cytokine-based therapies, adoptive cell therapies, and antibody blockade of negative regulatory pathways such as cytotoxic T lymphocyte-associated protein 4 (CTLA-4) and programmed death (PD)-1/L1 have demonstrated the effectiveness of immune-stimulating agents for cancer treatment.1 In both humans and mice, the immune system also surveys for nascent cancer development, although the extent to which immune surveillance happens in the absence of therapy is difficult to quantify.