Probably involved in the control of the cell cycle. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Phosphorylation at Thr-33 or Tyr-34 inactivates the enzyme. Forms a stable but non-covalent complex with a regulatory subunit and with a cyclin. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.