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Berkshire Hathaway Chairman and CEO Warren Buffett.

Citigroup shares got a boost from Warren Buffett who just disclosed a big stake in the struggling bank.

The bank stock jumped 7.7% on Tuesday after a filing revealed Monday evening that Buffett's Berkshire Hathaway added more than 55 million shares to build a stake worth $2.95 billion as of the end of March.

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