In addition to increasing in volume, hypertrophic chondrocytes secrete ECM that is later mineralized. These cells then die, leaving behind a calcified matrix that is eventually broken down, allowing invasion of blood vessels and XL880 c-Met inhibitor infiltration of bone-resorbing and bone-forming cells. Many key regulators have been shown to be required for cartilage and bone forming processes, including the transcription factor Sox9. Sox9 is an SRY-related hydroxymethylglutaryl box member of the Sox family and has been shown to regulate many developmental events . In the developing cartilage, Sox9 is expressed in immature chondrocytes and its function is required for mesenchyme cell condensation, proliferation and PD 0332991 CDK inhibitor differentiation . This positive role of Sox9 during early stages of chondrogenesis is largely attributed to its ability to bind and transactivate SRY consensus sites within cartilage ECM genes; these include Col2a1, Col9a2, Col11a2, Col27a1 and the proteoglycans Aggrecan and Cartilage Link Protein . In vivo, mice with reduced Sox9 function fail to develop cartilage and mimic the human skeletal disorder, Campomelic Dysplasia , further supporting a positive role for Sox9 in promoting formation of cartilaginous connective tissue. Following chondrocyte maturation, Sox9 expression is downregulated and levels remain low during stages of terminal differentiation and matrix mineralization . Enforced expression in mature chondrocytes results in delayed endochondral ossification , suggesting that Sox9 has a repressive role in bone formation. Consistently, Sox9 haploinsufficiency leads to premature mineralization in many skeletal tissues . Despite these observations, the mechanisms of Sox9 function in bone are not fully understood. In contrast to Sox9, expression of the transcription factor Runx2 is high in mature chondrocytes and osteoblasts . It has previously been shown that Sox9 binds RUNX2 to prevent transactivation of RUNX2 target genes, including the glycoprotein, Spp1 , thereby repressing bone formation. An additional mechanism of Sox9-mediated repression of osteogenic processes includes findings by Hattori et al., showing that Sox9 negatively regulates Vegfa to inhibit cartilage vascularization and subsequent mineralization. Together these previous studies have revealed pivotal roles for Sox9 in regulating formation of cartilage and bone connective tissues in the skeletal system. In addition to the skeletal system, we, and others have shown that Sox9 is also expressed in the developing heart .