The pathophysiological process of AD is multifaceted, involving several key hypotheses: the cholinergic, glutamatergic, amyloid, tau protein, inflammatory, microbiota–gut–brain axis, oxidative stress, metal ion, and abnormal autophagy hypotheses, as shown in the summary of AD pathophysiology in Figure 1 [8]. The gene discussed is MAPT; the disease is Alzheimer disease.