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). {ECO:0000269|PubMed:10574912, ECO:0000269|PubMed:22012619, ECO:0000269|PubMed:9632813}. This is the function of CCNK (cyclin K, Ensembl gene identifier ENSG00000090061).