Virus-mediated delivery of a gene that encodes the dopamine-synthesizing enzyme l-amino acid decarboxylase (AADC) to the putamen can improve the response to levodopa therapy in patients with Parkinson disease (PD), according to research published in Movement Disorders. In many people with PD, AADC levels diminish over time, leading to loss of efficacy of levodopa. In a cohort of 13 patients with PD, John Nutt and colleagues observed improvements in motor responses to two different levodopa doses (0.6 mg/kg/h and 1.2 mg/kg/h) ~6 months after AADC gene therapy. The improvements were especially pronounced with the 0.6 mg/kg/h dose, suggesting that AADC gene therapy could enable the use of reduced levodopa doses, thereby making patients less susceptible to the adverse effects of this drug.