COL1A1 and osteogenesis imperfecta: The majority of OI cases are caused by mutations in COL1A1 and COL2A1 (the main fibrillar collagens expressed in bone) and in other genes encoding proteins that control type I collagen biosynthesis by osteoblasts.1,2 Notably, many genes that are important for osteoblast formation have been identified to cause skeletal fragility and phenotypes similar to collagen-mutated OI.3 Osteocytes are the most abundant and longest-lived cells in bone.