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What: Shares of Yandex (NASDAQ:YNDX) have tanked today by as much as 14% after the company reported worse-than-expected earnings.

So what: Revenue in the fourth quarter came in at $290.4 million, but that 37% growth came at a high price as marketing and product development costs soared and held back the bottom line. The earnings per share of $0.26 was significantly below the $0.66 per share in profit that analysts and investors were expecting.

Now what: Concerns over growth deceleration also rattled investors. Yandex sees 2013 revenue growing by 28% to 32% on a ruble basis, which is down from the 44% full-year sales growth that the Russian search engine saw in 2012. That guidance is also lower than the 33% growth that analysts were modeling for. CFO Alexander Shulgin said the ad market is maturing and Yandex won't be able to grow sales as much as it used to.

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