The 3-HAO family of proteins consists of intramolecular dioxygenases that contain non-heme ferrous iron. HAAO (3-hydroxyanthranilate 3,4-dioxygenase), also known as HAO or 3-HAO, is a 286 amino acid monomeric cytoplasmic protein that belongs to the 3-HAO family. Utilizing Fe2+ ions as a cofactor, HAAO participates in the formation of quinolinic acid (QUIN), and excitotoxin, by catalyzing the oxidative ring opening of 3-hydroxyanthranilate to 2-amino-3-carboxymuconate semialdehyde. HAAO may play a role in disorders that are associated with varying levels of QUIN expression, and is widely expressed in peripheral organs, including kidney and liver. Existing as two alternatively spliced isoforms, the gene encoding HAAO maps to human chromosome 2p21 and mouse chromosome 17 E4.