During development. Effect is age dependent. Deficient mice developed ectopic tendons and joint ossification, and osteoarthritis 3 months after birth. Injured Bgn−/− tendons are more cellular than WT. In Bgn/Fmod double null mice, prevented formation of mature collagen fibrils. Weaker tendons with decreased stiffness. Alteration in collagen fibril structure, larger diameter, and abnormal morphology. Altered mech. properties. This evidence concerns the gene BGN and osteoarthritis.