Activator of CDK5/TPKII. Heterodimer of a catalytic subunit and a regulatory subunit. Myristoylated. The Gly-2-Ala mutant is absent of the cell periphery, suggesting that a proper myristoylation signal is essential for the proper distribution of CDK5R2 (p39) (By similarity). Belongs to the cyclin-dependent kinase 5 activator family.