FUNCTION Structural component of hyaline cartilage and vitreous of the eye.SUBUNIT Heterotrimer of an alpha 1(IX), an alpha 2(IX) and an alpha 3(IX) chain.SUBCELLULAR LOCATION Covalently linked to the telopeptides of type II collagen by lysine-derived cross-links.PTM Prolines at the third position of the tripeptide repeating unit (G-X-Y) are hydroxylated in some or all of the chains.DISEASE The disease is caused by variants affecting the gene represented in this entry.DISEASE Disease susceptibility is associated with variants affecting the gene represented in this entry. Susceptibility to intervertebral disk disease is conferred by variant p.Arg103Trp (PubMed:11308397).DISEASE The disease is caused by variants affecting the gene represented in this entry.SIMILARITY Belongs to the fibril-associated collagens with interrupted helices (FACIT) family.