Shire Plc made an unsolicited offer to buy Baxalta Inc., a business spun off by Baxter International Inc. last month, for about $30 billion in stock to create a biotech company focused on rare diseases.

Shire offered Baxalta shareholders 0.1687 American depositary receipts for each share held, valuing the target company’s stock at $45.23, a 36 percent premium over yesterday’s close, it said in a statement on Tuesday. Baxalta soared 25 percent to $41.35 in early U.S. trading.

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