Involved in the control of the cell cycle. Interacts with cyclins A, B, D, or E. Activity of CDK2 is maximal during S phase and G2. 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-14 or Tyr-15 inactivates the enzyme, while phosphorylation at Thr-160 activates it (By similarity). Activated by spdya. Interacts with spdya. Synthesized in unfertilized egg, but no longer made in the early embryo. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.