Skin cells from an adult mouse reprogrammed to act like embryonic stem cells have silenced symptoms of Parkinson's disease in rats. Scientists injected healthy rats with a toxin that destroyed their dopamine-making neurons, producing motor symptoms reminiscent of Parkinson's. The rodents then received treatment with the modified cells (called induced pluripotent stem cells). Within four weeks most of the rats showed improved balance and coordination; one even had heightened dopamine activity. Still, many issues must be resolved before the procedure can be adapted for humans. For one thing, scientists have yet to exactly mimic Parkinson's in rodents because the disease is so complex. In addition, the retroviruses used to transform the skin cells are known cancer triggers. The findings, published online April 7 by the Proceedings of the National Academy of Sciences USA, nonetheless appear to mark the first time manipulated cells have integrated into brain tissue and reversed neurodegenerative damage.