Moreover, genetic deletion of Sox9 in cells expressing mutant IFITM5 led to improvement in the growth plate architecture and improved the low bone mass phenotype (Figure 8), supporting the hypothesis that SOX9 plays a critical role in the pathological mechanism of OI type V. SOX9 is a master transcription factor in chondrocyte differentiation (43, 44) and is downregulated in hypertrophic chondrocytes. The gene discussed is SOX9; the disease is osteogenesis imperfecta type 5.