SNCA and Parkinson disease: Increased total plasma/serum α-syn levels are observed in PD and primarily occur in the early phases of the disease (El-Agnaf et al., 2006; Tinsley et al., 2010); however, there are studies where plasma/serum concentrations of oligomeric and phosphorylated species of α-synuclein show no difference (Zubelzu et al., 2022).