Five of these (APLP2, EN2, IREB2, NGFB, SLC18A2) were not associated with Parkinson's disease or related disorders in several important genetic disorder databases, including the Online Mendelian Inheritance in Man (OMIM), Genetics Home Reference (GHR) [25], Genetic Association Database (GAD) [26], or the Parkinson Disease mutation database (MutPD) [27]. Here, NGF is linked to hereditary disease.