Accordingly, proper neuronal function relies upon tight regulation of Cdk5 activity, and alterations in Cdk5 activity are associated with many age-related neurodegenerative diseases, including Alzheimer disease, Parkinson disease, and amyotrophic lateral sclerosis (Nguyen et al., 2001; Patrick et al., 1999; Qu et al., 2007; Smith et al., 2003). Here, CDK5 is linked to Parkinson disease.