CDKs (Cyclin-dependent kinases) are serine-threonine kinases first discovered for their role in regulating the cell cycle. They are also involved in regulating transcription, mRNA processing, and the differentiation of nerve cells. CDKs are relatively small proteins, with molecular weights ranging from 34 to 40 kDa, and contain little more than the kinase domain. In fact, yeast cells can proliferate normally when their CDK gene has been replaced with the homologous human gene. 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.

There are around 20 Cyclin-dependent kinases (CDK1-20) known till date. CDK1, 4 and 5 are involved in cell cycle, and CDK 7, 8, 9 and 11 are associated with transcription.

CDK levels remain relatively constant throughout the cell cycle and most regulation is post-translational. Most knowledge of CDK structure and function is based on CDKs of S. pombe (Cdc2), S. cerevisia (CDC28), and vertebrates (CDC2 and CDK2). The four major mechanisms of CDK regulation are cyclin binding, CAK phosphorylation, regulatory inhibitory phosphorylation, and binding of CDK inhibitory subunits (CKIs).