These comprise defective antigen processing and presentation by downmodulation of major histocompatibility complex (MHC) expression as well as immune editing of the antigen repertoire of a given cancer.1 Upregulated inhibitory ligands, such as PD-L1, and secreted factors like indoleamine 2,3-dioxygenase (IDO, encoded by IDO1) functionally suppress antigen-presenting cells and cytotoxic cellular immune effectors.2, 3 In addition, cell-autonomous mechanisms may decrease susceptibility of cancer to immune effector mechanisms. The gene discussed is IDO1; the disease is cancer.