COL9A1 encodes the α1 chain of type IX collagen, a structural component of hyaline cartilage and the vitreous of the eye that contributes to extracellular matrix organization and tensile strength 1. The gene contains two promoter regions: an upstream promoter driving expression of a longer chain in cartilage, and a downstream promoter driving expression of a shorter chain in the cornea and vitreous 1. Chondrocyte-specific expression is regulated by SOX9, which binds to multiple SOX/Sry-binding sites within the proximal promoter region 1. During osteoarthritis, LSD1, a chr6 remodeling enzyme, negatively regulates COL9A1 expression through interaction with SOX9 at the gene promoter, contributing to cartilage matrix degradation 2. Clinically, elevated plasma COL9A1 levels are among the strongest predictors of osteoarthritis development, with hazard ratios of 1.54 (95% CI 1.48-1.61), and levels begin deviating from normal more than a decade before disease onset 3. Pathogenic variants in COL9A1 cause autosomal recessive Stickler syndrome, characterized by ocular, auditory, and skeletal abnormalities 4, and COL9A1 mutations are identified in short-stature patients with skeletal dysplasias 5. Gene-environment interactions between COL9A1 polymorphisms and maternal drinking influence susceptibility to congenital talipes equinovarus 6.