SNCA and Parkinson disease: Proteinopathies with α-synuclein (α-syn) aggregates form a heterogeneous group of diseases called “synucleinopathies.” Alpha-synuclein is deposited as intracellular inclusions (i) inside neurons in Parkinson's disease (PD), PD with dementia (PDD), and dementia with Lewy bodies (DLB) and (ii) inside oligodendroglial cells in multiple system atrophy (MSA).