SNCA and Parkinson disease: Despite the rarity of pathogenic SNCA variants, the relevance of α-synuclein to both monogenic and idiopathic forms of PD has been established by the presence of Lewy bodies, the altered expression of SNCA in neurons of people with idiopathic PD [42] and the SNCA loci identified in GWAS studies of idiopathic PD (see Section 3).