Because the suppression of Notch signal intensity will decrease the population of neural stem cells in the brain, an attractive explanation for age-dependent neurodegeneration in LRRK2- or α-synuclein-linked PD is that impaired Notch signaling diminishes the maintenance of neuronal progenitor cells in the adult brain, making the brain more vulnerable to neuronal damage. This evidence concerns the gene SNCA and Parkinson disease.