Background: Cyclin-dependent kinase 5 (Cdk5) is crucial for brain development. Results: In contrast to neurons that utilize p35 as the primary Cdk5 activator, oligodendroglia employ p39-dependent Cdk5 activation to advance differentiation and myelin repair. Conclusion: p39 is the primary Cdk5 activator in oligodendroglia, essential for oligodendroglia development. Significance: Our study revealed distinct mechanisms controlling Cdk5 activity in neurons and oligodendroglia. © 2013 by The American Society.