Cyclin-dependent kinase 5. Molecular model showing the secondary structure of the enzyme cylcin- dependent kinase 5 (cdk5). Unlike other cyclin- dependent kinases, cdk5 does not appear to have any involvement in regulating the cell cycle. Instead, it is expressed in neurons (nerve cells) where it is involved in the development of the central nervous system (CNS) and the formation of synapses, the junctions between nerve cells.