Cell division protein kinase 8 is a member of the cyclin-dependent protein kinase （CDK） family. CDK8 and cyclin C associate with the mediator complex and regulate transcription by several mechanisms. CDK8 binds to and/or phosphorylates several transcription factors, which can have an activating or inhibitory effect on transcription factor function. CDK8 phosphorylates the Notch intracellular domain, SREBP, and STAT1 S727. CDK8 also inhibits transcriptional activation by influencing turnover of subunits in the mediator complex tail module. In addition, CDK8 influences binding of RNA polymerase II to the mediator complex. CDK8 is a colorectal cancer oncogene: the CDK8 gene is amplified in human colorectal tumors, activating β-catenin-mediated transcription that drives colon tumorigenesis.