SNCA and Parkinson disease: Whereas the genetics of PD are complex with a contribution of Mendelian (e.g., SNCA, LRRK2, PARKIN, PINK1) and non-Mendelian factors (e.g., single nucleotide polymorphisms), more common environmental risk factors including diet and alterations of the gut microbiome are in the focus of recent PD research [14,15,16,17].