CDK5 and Alzheimer disease: p25 is an aberrant cyclin-dependent kinase 5 (Cdk5) activator generated from calpain-mediated cleavage of the Cdk5 activator p35 under neurotoxic conditions, and p25/Cdk5 is a proline-directed serine/threonine kinase implicated in several neurodegenerative diseases, including AD [15].