Patients with GDD have shown variation in the age of onset and the expression of disease phenotypes of the maxillofacial and long bones, even within the same family whose affected members all share the same mutation [3,8,11,12,14-25], which may result from mutations in potential modifier genes, including COL5A1and COL1A1 [15,26,27]. Interestingly, these genes are also related to osteogenesis imperfecta. The Ano5 mutation observed in a GDD patient knock-in mouse (Ano5KI/KI) replicated GDD-like skeletal features [28]. This evidence concerns the gene COL1A1 and osteogenesis imperfecta.