Dr. Roger C. Duvoisin, a neurologist who played a leading role in breakthroughs in treating Parkinson's disease and discovering the genetic roots of the disorder, has died. He was 90 and lived in Chapel Hill, N.C. Duvoisin was chairman of the department of neurology at Rutgers Robert Wood Johnson Medical School in 1996 when his team of clinicians and researchers, in collaboration with scientists at the National Institutes of Health, identified the gene mutation responsible for many cases of Parkinson's. The discovery revolutionized Parkinson's research and opened the way to new therapies that are being tested in clinical trials today. Earlier in his career, Duvoisin helped pioneer the use of L-dopa as a treatment for Parkinson's. Described as a "miracle drug," L-dopa marked a significant advance in providing relief from the tremors, rigidity and other symptoms of the disease.