IDO1 and neoplasm: IDO1 functions in suppression of anti-tumor immunity by degrading tryptophan and producing a series of toxic kynurenine metabolites to promote immune evasion of tumors (Uyttenhove et al., 2003; Munn and Mellor, 2013), but the inhibitor of IDO1 failed in a phase 3 clinical trial (Long et al., 2019).