Increased α-synuclein (α-syn) levels, which occur in rare cases with multiplication of the SNCA gene, are sufficient to cause PD (Singleton et al., 2003; Ibanez et al., 2004) and mutations associated with SNCA lead to an increase in aggregation propensity (Conway et al., 1998; Polymeropoulos et al., 1997; Kruger et al., 1998). Here, SNCA is linked to Parkinson disease.