DRD4 and Parkinson disease: For instance, studies have shown that the 5-HTTLPR polymorphism in the serotonin transporter gene is associated with an increased risk of depression, as evidenced in patients with Parkinson’s disease Similarly, variations in the dopaminergic system, such as the COMT Val108/158Met and DRD4 polymorphisms, have been linked to mood disorders, anxiety symptoms, and stress responses [51,52,53].