Note that Kyn should be considered to be an AHR ligand as well as a surrogate for downstream metabolites/AHR ligands, such as kynurenic acid [136], xanthurenic acid [108], cinnabarinic acid [137], and trace kynurenine derivatives [138], any of which could serve as the ultimate effector ligands in a given cancer type. Here, AHR is linked to cancer.