SNCA and Parkinson disease: The process of α-synuclein aggregation is dose-dependent as evidenced by the autosomal dominant heritability of PD in rare families with elevated levels of normal α-synuclein caused by multiplications of the α-synuclein gene SNCA (Singleton et al., 2003; Chartier-Harlin et al., 2004; Devine et al., 2011).