CDK9 protein, located at 9q34.11 and consisting of 372 amino acids, is a CDC2-related kinase and the catalytic subunit of the positive-transcription elongation factor b (P-TEFb). It was first identified in 1994 and is primarily involved in regulating the elongation phase of transcription by RNA polymerase II (RNAPII). CDK9 forms complexes with cyclin T1, T2a, T2b, or K, and these complexes phosphorylate the C-terminal domain (CTD) of RNAPII and negative-transcription elongation factors, thereby overcoming promoter-proximal pausing and facilitating transcription elongation. CDK9 is also involved in transcription initiation and termination, and its activity is regulated by post-translational modifications such as phosphorylation and acetylation. Additionally, CDK9 has been implicated in various cellular processes, including immune responses and metabolic regulation, and is a potential therapeutic target in cancer and other diseases.