Vertex Pharmaceuticals (NASDAQ: VRTX) fell by 13% on Dec. 19, when the rare-disease drug developer reported some lackluster results from a late-stage clinical trial. But far from being a reason to doubt the long-term potential of this stock,

Shares of Vertex Pharmaceuticals Inc. VRTX slid 0.46% to $408.76 Friday, on what proved to be an all-around rough trading session for the stock market, with the S&P 500 Index SPX falling 1.11% to 5,970.84 and Dow Jones Industrial Average DJIA falling 0.77% to 42,992.21. The stock's fall snapped a four-day winning streak.

Vertex Pharmaceuticals shares sank Thursday after the company reported mixed results in a Phase 2 drug trial to treat lumbosacral radiculopathy (LSR), which causes leg and back pain.

Vertex Pharmaceuticals (NASDAQ: VRTX) was on track to become one of this year's biotech winners, with the shares climbing 27% from January through a peak in November. The company, the global leader in cystic fibrosis (CF) treatments,

This was the stock's fourth consecutive day of gains.