In the framework of Precision Medicine, the patient’s assessment is aimed at not only providing a clinical diagnosis by proper in-vivo biofluid or neuroimaging techniques measuring disease-specific neuropathological species in the brain of PD or AD patients (e.g., amyloid-beta and tau in AD patients), but also to use reliable and valid biomarkers accounting for neurobiological, neuroanatomical, and neurophysiological underpinnings of the individual clinical manifestations along the disease course. The gene discussed is MAPT; the disease is Alzheimer disease.