What happened Shares of Gilead Sciences, Inc. (NASDAQ: GILD) slipped 7.5% last month, according to S&P Global Market Intelligence, reducing the biotech's market cap by $7.8 billion. Although the company reported some positive data for two key programs in development, hepatitis C virus (HCV) sales slid much further than expected. So what Gilead Sciences shocked the market in February with a disturbingly bleak sales forecast for its HCV franchise, which peaked at $19.1 billion in 2015. In July, management lifted investors' spirits by nudging the lower end of its 2017 forecast from $7.5 billion to $8.5 billion after total hepatitis C sales ticked a bit higher sequentially in the second quarter. Image source: Getty Images. The upward revision in July made Gilead look uncharacteristically silly when HCV sales fell to just $2.2 billion in the third quarter. That was lower than the $2.6 billion reported in the first quarter and far lower than the $2.9 billion showing in the second quarter. The surprise dip led Gilead to lower the upper end of the full-year HCV sales estimate it had revised upward just three months earlier. The lack of visibility forces biopharma investors to ask themselves, "If Gilead's highly regarded management team can't predict the effects of increasingly aggressive price competition a few months into the future, how accurate are our long-term predictions?" Now what If there's one thing that Wall Street hates, it's uncertainty. That's a big reason Gilead Sciences stock trades at just 8.0 times trailing-12-month earnings while the average stock in the benchmark S&P 500 index has been trading at about 24.2 times trailing earnings. Last month Gilead also announced positive results from a study that measured the effects of switching from an existing HIV regimen to its experimental combo. While the company's HCV revenue stream is drying up, a triple-combination HIV treatment under review by the Food and Drug Administration could top $5 billion in annual sales, if approved on or before the FDA's target action date, Feb. 12, 2018. 10 stocks we like better than Gilead SciencesWhen investing geniuses David and Tom Gardner have a stock tip, it can pay to listen. After all, the newsletter they have run for over a decade, Motley Fool Stock Advisor, has tripled the market.* David and Tom just revealed what they believe are the 10 best stocks for investors to buy right now... and Gilead Sciences wasn't one of them! That's right -- they think these 10 stocks are even better buys. Click here to learn about these picks! *Stock Advisor returns as of October 9, 2017Cory Renauer owns shares of Gilead Sciences. The Motley Fool owns shares of and recommends Gilead Sciences. The Motley Fool has a disclosure policy.