The Collagen Type IX protein (also known as Collagen α1 Type IX) is encoded by the COL9A1 gene which possesses two promoter regions and codes for both a long chain Collagen Type IX protein expressed in the cartilage, and a shorter Collagen Type IX protein expressed in the cornea and vitreous. Collagen Type IX forms a heterotrimer with Collagen α2 Type IX and Collagen α3 Type IX. When it is expressed in hyaline cartilage, Collagen Type IX posseses a large N-terminal globular domain (NC4). The COL9A1 gene is also expressed in the human inner ear, and disruption of this gene in mice results in hearing loss, indicating the role of Collagen Type IX in hearing. Mutations in the COL9A1 gene are associated with multiple epiphyseal dysplasia (MED), a chondrodysplasia, in humans. Collagen Type IX is often co-localized with Collagen Type II, and may play a role in the interaction of fibrils betweennCollagen Type II and Collagen Type IX.
