PD is currently being studied using three different methods: glutamate decarboxylase (GAD)-inhibited glutamine synthesis as a neurotransmitter; aromatic amino acid decarboxylase, AADC-induced dopamine production; and glial cell line-derived neurotrophic factor (GDNF) in the substantia nigra to protect nigral neurons. The gene discussed is GDNF; the disease is Parkinson disease.