Misfolded, aggregated forms of αSyn are the primary proteinaceous component of Lewy bodies, and missense mutations and duplication/triplication of the SNCA gene cause aggressive, early-onset forms of familial PD, strongly suggesting that αSyn plays a central role in the pathogenesis of PD and related synucleinopathies such as Lewy body dementia. This evidence concerns the gene SNCA and Parkinson disease.