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). Widely detected at one day post-fertilization (dpf) and continuously expressed in the brain through development. Morpholino knockdown of the protein causes unconsumed yolk sac, small eyes, deformed head, reduced lower jaw size and curly spinal cord at low morpholino doses. High morpholino doses lead to embryonic lethality. Belongs to the cyclin family.