Regulatory subunit of cyclin-dependent kinases that mediates activation of target kinases. Plays a role in transcriptional regulation via its role in regulating the phosphorylation of the C-terminal domain (CTD) of the large subunit of RNA polymerase II (POLR2A). Regulatory subunit of cyclin-dependent kinases. Identified in a complex with a kinase and the RNA polymerase II holoenzyme. Interacts with POLR2A. Interacts with CDK12 and CDK13. Interacts with CDK9 according to PubMed:10574912; does not interact with CDK9 according to PubMed:22012619. Widely expressed. Highest levels in testis. The disease is caused by variants affecting the gene represented in this entry. Belongs to the cyclin family. Cyclin C subfamily.