Cyclin-dependent kinase-5 (CDK5) is a member of the cyclindependent kinase family of serine/threonine kinases. Its mRNA and protein are expressed in kidney, testes, and ovary. And Its activity is detected almost exclusively in brain extracts. Similar to other Cdks, monomeric Cdk5 displays no enzymatic activity, but Cdk5 is not activated by cyclins. Instead, Cdk5 activity requires association with one of two brain-specific regulatory subunits called p35 and p39. The two activators regulate the spatial and temporal expression of active Cdk5 to restrict its activity primarily to post-mitotic neurons.