Structural component of hyaline cartilage and vitreous of the eye. Heterotrimer of an alpha 1(IX), an alpha 2(IX) and an alpha 3(IX) chain. Covalently linked to the telopeptides of type II collagen by lysine-derived cross-links. Prolines at the third position of the tripeptide repeating unit (G-X-Y) are hydroxylated in some or all of the chains. The disease is caused by variants affecting the gene represented in this entry. 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). Belongs to the fibril-associated collagens with interrupted helices (FACIT) family.