In this proposal we will study roles of cyclin-dependent kinase 12 (CDK12) in the regulation of RNA Polymerase II (RNAPII) transcription. CDK12 activity is required for optimal elongation rate and processivity of RNAPII on protein coding genes. We and others demonstrated that CDK12 inhibition changes phosphorylation of the C-terminal domain (CTD) of RNAPII, reduces RNAPII elongation speed and leads to premature termination of transcription. However, molecular mechanism(s) by which CDK12 regulates transcription remains unclear. CDK12 is also frequently deregulated in various cancer types and is a potential drug target, therefore determining its molecular mode-of-action is important. In our preliminary results we identified new CDK12-specific substrates and proteins those association with RNAPII depends on the CDK12 kinase activity. We propose further characterization of these new players with a goal to obtain insight into the mechanisms of CDK12-mediated regulation of transcription.