Forms a stable complex with cyclin CYCH1-1 that phosphorylates human CDK2 and the C-terminal domain (CTD) of the large subunit of RNA polymerase II. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] [DNA-directed RNA polymerase] + ATP = ADP + H(+) + phospho-[DNA-directed RNA polymerase] Activated by phosphorylation by CDKF-1. Down-regulated by phosphorylation by WEE1. Interacts with CYCH1-1. Binding to CYCH1-1 activates CDK kinase. Expressed in suspension cell culture, but not in plant organs. Phosphorylated by CDKF-1 at Ser-162 and Thr-168. Phosphorylated by WEE1 at Tyr-24. Autophosphorylated. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.