SNCA and Parkinson disease: Numerous findings indicate that pathologically aggregated species of amyloid-β (Aβ) and tau protein in Alzheimer’s disease (AD), and of α-synuclein (αSyn) in Parkinson’s disease (PD) can act as proteinaceous nuclei (‘seeds’) which recruit endogenous precursor proteins and convert them into their own misfolded oligomeric or polymeric aggregate structures.