AHR and cancer: Among AhR's natural ligands, tryptophan derivatives such as 6-formylindolo[3,2-b]carbazole (FICZ)12, Kynurenine (Kyn)6 and 2-(1′H-indole-3′-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE)13 have received increasing attention for their emerging roles in cancer immunology.