Cyclin-dependent kinase that phosphorylates the C-terminal domain (CTD) of the large subunit of RNA polymerase II (POLR2A), thereby acting as a key regulator of transcription elongation. Regulates the expression of genes involved in DNA repair and is required for the maintenance of genomic stability. Preferentially phosphorylates 'Ser-5' in CTD repeats that are already phosphorylated at 'Ser-7', but can also phosphorylate 'Ser-2'. Involved in regulation of MAP kinase activity, possibly leading to affect the response to estrogen inhibitors (By similarity). Required for RNA splicing, possibly by phosphorylating SRSF1/SF2. [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] Interacts with CCNL1 and CCNL2. Colocalized with nuclear speckles throughout interphase. Expressed in embryonic tissues such as brain, spinal cord, heart, lung, liver, gut and limb. Levels are lower in adult tissues. Expressed at 10.5 and 14.5 dpc. Phosphorylation at Thr-889 increases kinase activity. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.