PRRX1 and osteogenesis imperfecta type 5: Conditional expression of mutant Ifitm5 in the Rosa26mIfitm5 Prx1-Cre and Rosa26mIfitm5 Acan-Cre ERT2 mice resulted in a low bone mass phenotype, as seen in individuals with OI type V. Biomechanical testing in the Rosa26mIfitm5 Acan-Cre ERT2 model demonstrated reduced ductility and increased bone stiffness, correlating with the findings in other OI mouse models (55, 56).