The underlying genetic basis of PD is complex: a small proportion of PD cases are accounted for by monogenic forms of PD inherited in a Mendelian fashion (e.g., related to mutations in α-synuclein (SNCA), leucine-rich repeat kinase 2 (LRRK2), parkin, Parkinson’s disease-associated kinase-1 (PINK-1), DJ-1, ATP13A2), but numerous susceptibility genes and loci have also been identified (at least 24) (9), which contribute to the risk of idiopathic PD. Here, SNCA is linked to Parkinson disease.