Inhibition of CTLA-4 (cytotoxic T-lymphocyte-associated protein 4), PD-1 (programmed cell death 1) and IDO1 (indoleamine 2,3-dioxygenase 1) has demonstrated antitumor efficacy in preclinical models and humans across several types of cancers [1–10]. Here, CTLA4 is linked to cancer.