Background：Cyclin-dependent kinases （CDKs）are a family ofproteinkinasesfirst discovered for their role in regulating thecell cycle. They are also involved in regulatingtranscription, mRNA processing, and the differentiation of nerve cells. They are present in all knowneukaryotes, and their regulatory function in thecell cyclehas been evolutionarily conserved. CDKs are relatively small proteins, witHuman, Mouseolecular weights ranging from 34 to 40 kDa, and contain little more than the kinase domain. By definition, a CDK binds a regulatory protein called a cyclin. Without cyclin, CDK has little kinase activity; only the cyclin-CDK complex is an active kinase. CDKs phosphorylate their substrates on serines and threonines, so they are serine-threonine kinases.
