Catalytic domain of the Serine/Threonine Kinase, Cyclin-Dependent protein Kinase 9STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. CDK9, together with a cyclin partner (cyclin T1, T2a, T2b, or K), is the main component of distinct positive transcription elongation factors (P-TEFb), which function as Ser2 C-terminal domain kinases of RNA polymerase II. P-TEFb participates in multiple steps of gene expression including transcription elongation, mRNA synthesis, processing, export, and translation. It also plays a role in mediating cytokine induced transcription networks such as IL6-induced STAT3 signaling. In addition, the CDK9/cyclin T2a complex promotes muscle differentiation and enhances the function of some myogenic regulatory factors. CDKs belong to a large family of STKs that are regulated by their cognate cyclins. Together, they are involved in the control of cell-cycle progression, transcription, and neuronal function. The CDK9 subfamily is part of a larger superfamily that includes the catalytic domains of other protein STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.