Approximately 90% of kynurenine is generated by indoleamine 2,3-dioxygenase (IDO), and the level of kynurenine is increased in the liver after CCl4 treatment accompanied by the activation of AhR signaling, whereas IDO2 inhibition attenuates hepatic fibrosis, hinting that kynurenine produced by IDO2 exacerbates liver injury by activating AhR [107]. Here, IDO2 is linked to Hepatic fibrosis.