An enzyme produced in the brain that synthesizes gamma-AMINOBUTYRIC ACID (GABA), an AMINO acid that functions as a neurotransmitter in the brain to reduce the sensitivity of muscle cells to nerve stimulation. GABA level decreases with dopamine depletion, which researchers believe contributes to the tremors and bradykinesia characteristic of Parkinson’s disease. Increasing the brain’s production of GAD consequently increases the action of GABA, helping to reduce motor symptoms and normalize movement. Researchers are experimenting with injecting the GAD gene, transported via an engineered virus structure, directly into the subthalamic nuclEus (STN). The STN is a component of the basal ganglia that produces much of the brain’s GABA. GAD gene injection appears to “turn on” the STN’s production of GABA. The U.S. Food and Drug Administration (FDA) approved the first phase i clinical research studies involving a small group of human volunteers, all younger than age 65 with advanced Parkinson’s disease that no longer responds to other treatments, in late 2002.