IDO1 and breast neoplasm: One possibility is that the increased expression and activity of AhR found in breast tumor cells causes altered levels of tryptophan metabolizing enzymes as shown for indoleamine 2,3-dioxygenase (IDO) and tryptophan-2,3-dioxygenase (TDO) (34, 44, 45) and generate high levels of the endogenous AhR ligand kynurenine (Kyn).