Several large shareholders of BankUnited (BKU) in Miami Lakes, Fla., are preparing to sell about one-fifth of the company's outstanding stock.

The $12.2 billion-asset company said Wednesday that certain shareholders are preparing an underwritten offering of 19.6 million shares of common stock in the company, or just under 21% of the total outstanding. The underwriters of the offering, Morgan Stanley and BofA Merrill Lynch, will have the option to buy an additional 2.9 million shares, which would bring the total offering to approximately $644 million at the company's share price of $28.57 Wednesday morning.

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