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 (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.

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.