Despite a lack of molecular mechanistic analyses, we speculate, in light of our current data, that AD and PD (and other less frequent neurodegenerative diseases) share not only “well-known” pathophysiological mechanisms (e.g., disturbed mitophagy, retromer, and proteasome functions), but also epigenetic mechanisms, such as A1 rs708727-dependent regulation of PM20D1 expression [27,42]. The gene discussed is PM20D1; the disease is neurodegenerative disease.