When the enzyme GAD is injected into the brain of a patient with Parkinson's, the production of a neurotransmitter relieves the patient's symptoms. "Typically, people with Parkinson's produce too little GABA (gamma-aminobutyric acid), and consequently have overstimulation in an area of the brain called the subthalamic nucleus. This overactivity in turn puts strain on neurons that produce another neurotransmitter—dopamine—which is vital for movement control. This helps explain some of the symptoms of Parkinson's, which include tremors, sluggish movements, rigid muscles and impaired posture and balance."