Most recently, the Cre/lox recombination system was used to develop a Col1a1 G349C substitution to reproduce a human OI type IV (moderately severe) phenotype termed BrtlIV.(14–17) The BrtlIV mouse represented a major advance in OI mouse models because it is the only Col1a1 model with a triple-helical glycine mutation. This evidence concerns the gene COL1A1 and osteogenesis imperfecta.