CDK9 (cyclin-dependent kinase 9, serine/threonine-protein kinase PITALRE) is a member of the cell division cycle 2 (CDC2) family of kinases that play a pivotal role in the regulation of the eukaryotic cell cycle. CDK9 expression is ubiquitous, but its expression levels are different in various human tissues. CDK9 is localized primarily to the nucleus. At pathophysiological levels, CDK9 activity suppresses many genes for mitochondrial proteins including master regulators of mitochondrial function (peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1), nuclear respiratory factor-1). Chronic activation of CDK9 causes not only cardiomyocyte enlargement but also defective mitochondrial function. The cyclinT/CDK9 complex, also called positive transcription elongation factor b (p-TEFb), phosphorylates the C-terminal domain of the large subunit of RNA polymerase II, stimulating transcription elongation and pre-mRNA processing.