Cell division protein kinase 5 is an enzyme that in humans is encoded by the CDK5 gene. The protein encoded by this gene is part of the cyclin-dependent kinase family. CDK5 is required for proper development of the brain and to be activated, CDK5 must associate with CDK5R1 or CDK5R2. Cdk5 is involved in the processes of neuronal maturation and migration, phosphorylating the key intracellular adaptor of the reelin signaling chain. Experiments performed on mice lacking p35, a necessary activator of cdk5 in early brain development, showed that the normal layering of neurons was reversed in the cortex. This disrupted lamination again implicated cdk5 in neuronal migration and plasticity. Cdk5 is also involved in the regulation of synaptic vesicle exocytosis via phosphorylation of munc-18.