COL9A1 and osteochondritis dissecans: As said, OCD has already been reported in COL9A2 MED; COL9A2 mutations cluster in specific regions, and the pathogenic mechanism in COL9-MED is poorly understood; however, it seems to be different from that in COMP-MED as it affects the composition, structure, and function of the cartilage extracellular matrix and does not cause ER stress [20,21].