Why do some people with multiple sclerosis (MS) continue to run marathons while others are in wheelchairs years after diagnosis? Genetics partly explains why. Scientists have identified the first genetic marker for MS severity, which could pave the way for new treatments to prevent disease progression. Sergio Baranzini of University of California San Francisco and Adil Harroud of McGill University in Canada discuss their findings with host Brett Drummond of MS Translate.