COL1A1 and osteogenesis imperfecta: Mutations in the encoding genes, COL1A1 and COL1A2, respectively, lead to qualitative or quantitative defects in the protein and give rise to osteogenesis imperfecta (OI), a skeletal dysplasia characterized by low BMD and enhanced bone fragility, and often extra-skeletal features, such as blue sclerae, dentinogenesis imperfect, and hearing loss (86, 87).