Taking into account the results of Alanay and colleagues and Christensen and colleagues, there are now three classes of mechanisms for generating an OI phenotype.(5,6) First, mutations in the COL1A1 or COL1A2 gene may cause quantitative and/or qualitative defects in type I procollagen leading to dominantly inherited OI.(22,23) Second, posttranslational modification of type I procollagen is also critical to forming properly functioning collagen helices. The gene discussed is COL1A1; the disease is osteogenesis imperfecta.