More than 95% of disease-causing mutations for OI have been found in COL1A1 (MIM# 120150) and COL1A2 (MIM# 120160), which encode the chains of type I procollagen, the major protein of bone (unpublished data and database of OI mutations [http://www.le.ac.uk/ge/collagen/]). The gene discussed is COL1A1; the disease is osteogenesis imperfecta.