In the following video, Motley Fool health-care analyst David Williamson explains what's behind a massively important trial for Vertex Pharmaceuticals (NASDAQ:VRTX). The company's cystic fibrosis drug Kalydeco currently treats only about 4% of the population, but Vertex is looking to apply the drug to a more common mutation of the disease, which would expand the drug's reach to be effective for nearly half of the CF patient population. David tells investors just how much money is at stake in this market, and what's at stake if the trial doesn't go as planned.

David Williamson has no position in any stocks mentioned. The Motley Fool recommends Gilead Sciences and Vertex Pharmaceuticals. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.