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. Its interaction with CDK9 is controversial. Ubiquitously expressed. Highest levels in testis. Present throughout the seminiferous epithelium. Also highly expressed in the developing oocyte. Found in 12.5 dpc embryo with abundant expression in eye (retinal pigment epithelium) and ear (cochlea, crista ampullaris of the semicircular canals). Complete embryonic lethality. No homozygous embryos are detected at any stage of embryonic development. Belongs to the cyclin family. Cyclin C subfamily.