Source: Gilead Sciences via Google Maps

Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares in Gilead Sciences (NASDAQ:GILD) dropped as much as 10% earlier today after the company indicated that despite reporting better than expected fourth-quarter earnings and announcing that it would begin paying a $0.43 quarterly dividend, its growth would slow this year. 

So what: Historically, delivering a quarter in which sales jump 137% and earnings per share more than quadruples would be a good thing, especially given that those results outpaced analyst forecasts. But in this case, investors hit the sell button on expectations that revenue and earnings growth will dip in 2015.

Overall, Gilead Sciences' revenue totaled $7.3 billion in the fourth quarter and $24.9 billion for the full year. That sales surge was primarily due to the success of the company's hepatitis C drugs Sovaldi, which won approval in December 2013, and Harvoni, which combines Sovaldi with ledipasvir and won FDA approval for use in genotype 1 patients in October. Sales of Sovaldi totaled $1.7 billion in the fourth quarter and $10.3 billion for the full year, while sales of Harvoni totaled $2.1 billion during its first quarter on the market.

Gilead Sciences also reported solid results for its multibillion-dollar HIV drug franchise. Sales of HIV drugs reached $2.9 billion in the fourth quarter, up 15.5% year over year, and $10.3 billion in 2014, up 13.2% from $9.1 billion in 2013. Revenue from the company's cardiovascular drugs Letairis and Ranexa also added $1.1 billion in sales last year.

Although Gilead Sciences expects demand to remain strong for its hepatitis C and HIV drugs this year, the company expects that price discounts, competition, and slowing growth in prescription volume will lead to sales of between $26 billion and $27 billion in 2015. At the low end, that would represent an increase of just 4.4% from the company's lofty 2014 levels.

Now what: Without a doubt, Gilead Sciences is unlikely to see its sales and profit grow anywhere near as fast as they did last year and that slowing in its top line growth could mean that some short-term growth investors might focus on other biotech stocks this year. However, long haul investors might want to keep their eye on the big picture. The company has very little patent risk to its product portfolio, has a stable of market share leading compounds, is sitting on a cash stockpile that has grown from $2.6 billion to $11.7 billion in the past year, and is now one of just two investment-quality, dividend-paying biotech stocks. Since the share drop has pushed its valuation to less than nine times 2015 projected EPS, investors might want to consider this sell-off as an opportunity to buy this leader in long term portfolios.