The main conclusions of this work are as follows: (i) total α-syn was demonstrated to interact with Aβ1–42 and tau in RBCs of PD patients; (ii) α-syn-Aβ1–42 heterocomplex concentrations were elevated in PD patients, and directly correlated with disease severity and motor deficits in patients under therapy; (iii) additionally, PD patients presented decreased concentrations of total α-syn and increased levels of its oligomeric form, as well as of phosphorylated tau; and (iv) total α-syn levels were inversely related to motor deficits of PD patients. This evidence concerns the gene MAPT and Parkinson disease.