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NEW YORK (TheStreet) -- KaloBios Pharmaceuticals (KBIO)  stock is spiking by 33.31% to $35.50 in morning trading Friday after CEO Martin Shkreli tweeted on Thursday that he would stop lending stock to people hoping to short it. 

"I spoke with my counsel & advisers and decided to stop lending my KBIO shares out until I better understand the advantages of doing so," Shkreli tweeted yesterday.

About 49% of KaloBios stock is short, MarketWatch reports.

To short a stock, an investor borrows it from someone who is long and later repurchases the shares. Short interest in a stock indicates that investors are betting that it will retreat further, so they can profit when they buy back shares at a lower price.

Those who have shorted KaloBios stock will attempt to cover their positions, giving Shkreli and others who are long an advantage, MarketWatch adds.

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KaloBiosis a biopharmaceutical company based in San Francisco. 

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