IDO1 and neoplasm: The most known agonist of AHR is 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD); however, more importantly for the field of tumor immunology, is the endogenous agonist kynurenine, which originates from tryptophan, in a reaction catalyzed by indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase 2 (TDO2) [14].