In what's being called a major breakthrough in the battle against Parkinsons disease, researchers at the University of Colorado School of Medicine have discovered a drug that stops the progression of the degenerative illness in mice.The drug phenylbutyrate is now being tested in humans.Phenylbutyrate turns on a gene that can protect dopamine neurons in Parkinsons disease, said Wenbo Zhou, PhD, assistant professor of medicine."Drugs currently used to treat Parkinsons disease just treat symptoms; they do not stop the disease from getting worse," said Dr. Curt Freed in a statement. "Weve now discovered that we can prevent the progression of the disease by turning on a protective gene in the brain."Aging mice receiving the drug were able to move normally, had no decline in mental function, and their brains did not accumulate the protein that causes Parkinsons, officials said. By contrast, older animals that did not get the drug saw a steady decline in their ability to move as their brains were damaged by abnormal proteins.The results are on the Journal of Biological Chemistry website.The researchers began giving phenylbutyrate to people in 2009, to test the safety of the drug in Parkinson's patients.Zhou and Freed will publish the human results in the coming months.