HAAO and hepatocellular carcinoma: Thus, the endogenous 3-HAA level is downregulated in sorafenib - resistant HCC cells, possibly due to the downregulation of kynurenine 3-monooxygenase (KMO) and kynureninase (KYNU), enzymes converting kynurenine into 3-HAA, and/or the upregulation of hydroxyanthranilate 3,4-dioxygenase (HAAO), enzyme degrading 3-HAA.