RUNX2 and osteogenesis imperfecta type 5: The roles of MEF2C and NFATc1/c2 in skeletal biology, studied in several bone cell types (osteoblast, chondrocytes, osteocytes, and osteoclasts), could in part explain the variable cellular and tissue anomalies, and phenotypes, observed in OI type V. MEF2C is a transcription factor involved in osteoblast differentiation and gene regulation, thought to be upstream of bone-specific transcription factors RUNX2 and Osterix (OSX/SP7) [53].