Cyclin-dependent kinase that acts as a master regulator of the mitotic and meiotic cell cycles (By similarity). Required to drive the G1-S transition (PubMed:2664468). More than 200 substrates have been identified (PubMed:14574415). Substrate specificity is in part regulated by the bound cyclin protein (By similarity). Phosphorylates YTA7 during S-phase to promote transcription of histones (PubMed:22156209). 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-18 or Tyr-19 inactivates the enzyme, while phosphorylation at Thr-169 activates it. Forms a stable but non-covalent complex with the CKS1 protein and with a cyclin. Present with 6670 molecules/cell in log phase SD medium. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.