For further investigating the characteristics of salivary total α-synuclein in different groups, bivariate correlation analysis revealed that salivary α-synuclein levels decreased with age in PD patients (r = −0.162, p = 0.021; r = −0.211, p = 0.003 when normalized, Fig. 1B), but not in healthy controls (p = 0.32, p = 0.74 when normalized, Fig. 1B). This evidence concerns the gene SNCA and Parkinson disease.