Cyclin-dependent kinase which displays CTD kinase activity: hyperphosphorylates 'Ser-2' in the C-terminal heptapeptide repeat domain (CTD) of the largest RNA polymerase II subunit, thereby acting as a key regulator of transcription elongation. Required for normal reproduction. [DNA-directed RNA polymerase] + ATP = ADP + H(+) + phospho-[DNA-directed RNA polymerase] ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Phosphorylation of 'Ser-2' of the RNA polymerase II C-terminal domain is undetectable in primordial germ cells and reduced by 60% in embryonic somatic nuclei. RNAi treatment causes sterility. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.