CDK5R1 and Alzheimer disease: Indeed, the vast majority of past and present reports on CDK5 have detailed its many roles in neuronal tissue and neurodegenerative diseases such as Alzheimer disease, in which it is believed that calpain-mediated cleavage of p35 to p25 leads to CDK5/p25 hyperactivity and phosphorylation of tau protein (14, 15).