The causes of AD are poorly understood [32], although several etiological factors, such as genetic abnormalities [33], history of head injuries, environmental factors, general lifestyles [34], depression or hypertension [32], deposition of extracellular β-amyloid protein (Aβ) and microtubule associated tau protein [33] in the brain, and cholinergic dysfunction have all been implicated in AD. The gene discussed is MAPT; the disease is major depressive disorder.