In the following video, Motley Fool health-care analyst David Williamson explains what's behind a massively important trial for Vertex Pharmaceuticals (VRTX 0.59%). 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.