Cyclin-dependent kinase 9 or CDK9 is a cyclin-dependent kinase associated with P-TEFb. The protein encoded by this gene is a member of the cyclin-dependent protein kinase (CDK) family. CDK family members are highly similar to the gene products of S. cerevisiae cdc28, and S. pombe cdc2, and known as important cell cycle regulators. This kinase was found to be a component of the multiprotein complex TAK/P-TEFb, which is an elongation factor for RNA polymerase II-directed transcription and functions by phosphorylating the C-terminal domain of the largest subunit of RNA polymerase II. This protein forms a complex with and is regulated by its regulatory subunit cyclin T or cyclin K. HIV-1 Tat protein was found to interact with this protein and cyclin T, which suggested a possible involvement of this protein in AIDS. CDK9 is also known to associate with other proteins such as TRAF2, and be involved in differentiation of skeletal muscle.

CDK9 has been shown to interact with Cyclin K,Cyclin T2,RELA,Cyclin T1,Retinoblastoma protein,Androgen receptor,SKP1A,MYBL2,CDC34 and SUPT5H.