Cyclin-dependent kinase  Cyclin-dependent kinases (CDKs) are the families of protein 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. They are present in all known eukaryotes, and their regulatory function in the cell cycle has been evolutionarily conserved. In fact, yeast cells can proliferate normally when their CDK gene has been replaced with the homologous human gene. CDKs are relatively small proteins, with molecular 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. The consensus sequence for the phosphorylation site in the amino acid sequence of a CDK substrate is [S/T*]PX[K/R], where S/T* is the phosphorylated serine or threonine, P is proline, X is any amino acid, K is lysine, and R is arginine. Most of the known cyclin-CDK complexes regulate the progression through the cell cycle. Animal cells contain at least nine CDKs, four of which, CDK1, 2, 3, and 4, are directly involved in cell cycle regulation. In mammalian cells, CDK1, with its partners cyclin A2 and B1, alone can drive the cell cycle. Another one, CDK7, is involved indirectly as the CDK-activating kinase. Cyclin-CDK complexes phosphorylate substrates appropriate for the particular cell cycle phase. Cyclin-CDK complexes of earlier cell-cycle phase help activate cyclin-CDK complexes in later phase. A list of CDKs with their regulator protein, cyclin or other:  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. cerevisiae" (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).