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. Each subunit is composed of three triple-helical domains interspersed with non-collagenous domains. The globular domain at the N-terminus of type IX collagen molecules represents the NC4 domain which may participate in electrostatic interactions with polyanionic glycosaminoglycans in cartilage. 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. Belongs to the fibril-associated collagens with interrupted helices (FACIT) family.