Down-regulating CDK9 (siCDK9) expression led to decreased cell proliferation, as well as reduced levels of pS2 RNAP II and MCL-1 whereas, treating the osteosarcoma cell lines with the ATP competitive CDK9 inhibitor LDC000067 significantly reduced the levels of pS2 RNAP II and MCL-1 and their clonogenicity, in a dose-dependent manner, in conjunction with increased apoptosis. The gene discussed is MCL1; the disease is osteosarcoma.