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.