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,

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.

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.

Different genetic mutations cause different problems with this particular protein -- so CFTR modulators generally don't work on every CF case. But Vertex's newest blockbuster, Trikafta -- approved in 2019 -- made a splash when it entered the market because it can be used for about 90% of patients with the disease.