MAPT and Parkinson disease: The levels of plasma total tau, Aβ42, and Aβ42/40 were significantly elevated in patients with PD compared with those in APD (total tau: PD vs. MSA: p < 0.01, PD vs. PSP: p < 0.05; Aβ42: PD vs. MSA: p < 0.0001, PD vs. PSP: p < 0.001; Aβ42/40: PD vs. MSA: p < 0.0001, PD vs. PSP: p < 0.0001; Figures 1A,B,D).