Cyclin-dependent kinase 5 (CDK5), a member of the serine/threonine kinase Cdk family, is complexed with p35 a neuron specific activator of Cdk5. The complex CDK5:p35 is required for neurite outgrowth and cortical lamination. CDK5:p35 complex is recruited to the growth cone by associating with active Fyn. FYN promotes the kinase activity of CDK5 by phosphorylating CDK5 on tyrosine residue 15. Activation of CDK5 by FYN via Tyr15 phosphorylation might facilitate suppression of RAC-PAK signaling downstream of PlexinA.