CDK9 and cancer: Moreover, BRD4’s interaction with the positive transcription elongation factor b (P-TEFb), which consists of Cyclin-dependent kinase 9 (CDK9), in tandem with one of the corresponding Cyclin T1, T2, or K, helps RNAPII escape the promoter-proximal pausing, allowing for the continuous transcription of genes critical to cancer progression [13].