This loop is autoregulated by AHR-dependent elevation of the AHR repressor, and CYP1A1 and CYP1B1 enzymes that metabolize kynurenine ligands to decrease their bioavailability [29], thus forming a cancer progression circuit where AHR-regulated CYP1B1 levels correlate with AHR activity. Here, CYP1A1 is linked to cancer.