Another gene therapy of PD involved insertion of the glutamic acid decarboxylase (GAD) into the subthalamic nucleus. GAD enzyme controls GABA productions. In Parkinson’s disease, the activity of both GABA efferents to the subthalamic nucleus and its target within the basal ganglia circuity are affected. This strategy used andeno-associated viral vectoe (AAV2) to deliver GAD to the subthalamic nucleus. The trial was done to compare the effect of bilateral delivery of AAV2-GAD into the subthalamic nucleus with bilateral sham surgery in patients with advanced Parkinson’s disease. The study showed the first success of randomised, double-blind gene therapy trial for a neurodegenerative disease and justified the continued development of AAV2-GAD for treatment of PD.