To slow down levodopa degradation, monoamine oxidase B (MAO-B) inhibitors are often included in the combination drug therapy for PD (Selegiline (5), Zonisamide (6)), as well as catechol-O-methyltransferase (COMT), which prevents the methylation of Levodopa to 3-methyldopa (Entacalone (7), Tolcalone (8)) (Figure 1) [12]. The gene discussed is COMT; the disease is Parkinson disease.