SNCA and Parkinson disease: Despite these associations, efforts to establish α-synuclein as a biomarker for PD diagnosis or prognosis primarily focus on three species of α-synuclein-total α-synuclein, phosphorylated α-synuclein, and the oligomeric form of α-synuclein-in tissues and fluids like blood components, CSF, saliva, and extracellular vesicle (EVs) (Hong et al., 2010; Tokuda et al., 2010; Mollenhauer et al., 2011, 2013; Foulds et al., 2012; Wang Y. et al., 2012; van Steenoven et al., 2018; Vivacqua et al., 2019; Stuendl et al., 2021).