IDO1 and neoplasm: Kyn, the main product of Trp metabolism pathway catalyzed by TDO2 and indoleamine 2,3-dioxygenase (IDO) in tumor cells, was demonstrated to activate aryl hydrocarbon receptor (AhR), suppressing antitumor immune responses and promoting tumor-cell survival and motility through AhR in an autocrine/paracrine fashion (6).