Parkinson’s disease (PD), Parkinson’s disease dementia (PDD), Lewy body dementia (LBD), multiple system atrophy (MSA) and several rarer diseases are together classified as ‘synucleinopathies’, a class of common neurodegenerative diseases characterized by the pathogenic accumulation of the 140 amino acid protein, α-synuclein (α-syn) in a subset of neuronal and glial cells. This evidence concerns the gene PROS1 and Parkinson disease.