COL1A1 and osteogenesis imperfecta: Although discoveries of causative mutations in over a dozen different genes have generated considerable excitement and identified new biochemical pathways that contribute to OI [3–9], about 90% of individuals with OI have a mutation in one of the two genes, COL1A1 and COL1A2, that encode the proα1(I) and proα2(I) chains of type I collagen, the major protein of bone [7,10].