MAO-B levels are enhanced in the brains of patients with Parkinson’s disease as a result of gliosis, since the human basal ganglia have elevated MAO-B than MAO-A expression, and because dopamine is uniformly metabolized by both isoenzymes in humans, the selective MAO-B inhibitor selegiline was earliest examined as an adjunct to levodopa. Here, MAOA is linked to Parkinson disease.