HAAO (3-Hydroxyanthranilate 3,4-dioxygenase) is a monomeric cytosolic protein of the family of intramolecular dioxygenases containing non-heme ferrous iron. It is widely distributed in peripheral organs, such as liver and kidney, and is present in low amounts in the central nervous system. This enzyme participates in tryptophan metabolism. It employs one cofactor, iron. HAAO catalyzes the synthesis of quinolinic acid (QUIN) from 3-hydroxyanthranilic acid. QUIN is an excitotoxin whose toxicity is mediated by its ability to activate glutamate N-methyl-D-aspartate receptors. Increased cerebral levels of QUIN may participate in the pathogenesis of neurological and inflammatory disorders. HAAO has been suggested to play a role in disorders associated with altered tissue levels of QUIN. Furthermore, recent study shows that HAAO are excellent candidate biomarkers for detecting ovarian cancer.