IDO2 and neoplasm: These include the production of arginase-1 (Arg-1), inducible nitric oxide synthase (INOS), indoleamine 2,3-dioxygenase (IDO) [5, 6] and a variety of soluble inhibitors that activate and amplify regulatory T cells (Tregs), so that tumor cells can escape immune surveillance, attack the body and promote tumor development.