CDKs are the kinases that add the phosphate groups to their target proteins. These proteins are present in an inactive form and are activated only when cyclins bind to them. The complex formed by cyclins and the respective CDKs activates the later. The activated CDKs then phosphorylate the target proteins. Some proteins are activated while the others become inactive upon phosphorylation. This is how CDKs regulate the progress of cells from one stage of the cell cycle to the other.
