PD is a multifactorial disease, so despite role of environmental exposure, genetic predisposition in sporadic cases can be broadly grouped into four categories: genes involved in the metabolism of xenobiotics (e.g., CYP2D6, NAT2, GSTs), neurodegeneration (e.g., NOS), the functioning of dopaminergic neurons (e.g., dopamine transporters and receptors), and linkage-derived genes (e.g., UCHL1, alpha-synuclein) [97]. Here, SNCA is linked to Parkinson disease.