Overview:

CDK9 is a member of the cyclin-dependent protein kinase (CDK) family. CDK9 is closely related to cdc28 and cdc2 and is an important regulator of the cell cycle (1). CDK9 is a component of the multiprotein complex TAK/P-TEFβ. CDK9 can modulate RNA polymerase II-directed transcription by phosphorylating the C-terminal domain of the largest subunit of RNA polymerase II. CDK9 forms a complex with and is regulated by its regulatory subunit cyclin T or cyclin K. CDK9 also interacts with the HIV-1 Tat protein which suggested a possible involvement of this protein in AIDS (2). 

References:

1. Yang, Z. et al: The 7SK small nuclear RNA inhibits the CDK9/cyclin T1 kinase to control transcription. Nature, 2001; 414: 317-322. 

2. Bullrich, F. et al: Chromosomal mapping of members of the cdc2 family of protein kinases, cdk3, cdk6, PISSLRE, and PITALRE, and a cdk inhibitor, p27-Kip1, to regions involved in human cancer. Cancer Res. 1995; 55: 1199-1205.