Proofpoint shares tumble 6.5% as short seller Carson Block names stock as best short

Shares of cybersecurity company Proofpoint Inc. tumbled 6.5% in afternoon trade Thursday, after well-known short seller Carson Block named the stock as his best short bet. Block, who heads Muddy Waters Research, made the call at a conference in London. The Sunnyvale, Calif.-based company's stock has gained 44% this year, easily outperforming the S&P 500, which is up about 1%. Block and his firm made headlines in 2011 with a short call on Sino Forest, a Toronto-listed lumber firm with operations in China. Block alleged accounting fraud. The company denied the allegations but eventually collapsed.

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