Additional existing lines of evidence that support this idea include the following: (1) depression is a common non-motor symptom of PD [67], (2) diagnosis of depression in individuals over the age of 50 increases the risk of developing PD [68], (3) bi-allelic mutations in PRKN cause early-onset PD [69], and (4) family members of individuals with early-onset PD carrying monoallelic PRKN mutations have a significantly elevated risk of experiencing depression compared with family members without PRKN mutations [70]. This evidence concerns the gene PRKN and depressive disorder.