Therefore, the aim of the present systematic review was to summarize available studies on salivary biomarkers for the diagnosis of neurodegenerative diseases, such as AD and PD, and to investigate whether differences regarding the concentrations of salivary biomarkers, amyloid-β and tau for AD and α-synuclein and DJ-1 for PD, can be observed in patients suffering from these neurodegenerative diseases compared to healthy individuals. The gene discussed is MAPT; the disease is Alzheimer disease.