Temporally orderly activation of cyclin-dependent kinases (Cdks) governs progression and transitions of the cell cycle in eukaryotic cells. Binding of Cdks to cyclins and threonine phosphorylation in the Cdks are required to form fully active holo-Cdks. So far, 8 types of cyclins and 7 Cdks are known in mammals. Two types of the cyclins, D-(D1, D2, D3) and E-type cyclins, function in the G1 phase of the cell cycle. D-type cyclins form active complexes with Cdk 4 or Cdk 6 earlier than E-type cyclin does with Cdk 2. Overexpression of the cyclin D1 has been reported in human tumors including parathyroid adenomas with chromosomal abnormality, suggesting that overexpression of the cyclin D1 could abrogate cell cycle control in G1 phase and may contribute to generate tumor cells.