CDK5 and neuroblastoma: Notably, overexpression of Cdk5 and p39 resulted in significantly higher rates of synapse formation in a neuroblastoma cell/myotube co-culture system [8], and both the knockout of Cdk5 [9] as well as the double-knockout of p39 and its homologue p35 in mice [10] lead to widespread disruption of neuronal migration and brain development.