Additionally, physiologically relevant levels of transgenic protein are being produced in the brain—of particular note is a study conducted by Manfredsson and colleagues who used an AAV/Tet-Off system to express glial cell line-derived neurotrophic factor at levels exceeding those required for efficacy in a 6-hydroxydopamine model of Parkinson's disease [7]. The gene discussed is GDNF; the disease is Parkinson disease.