Shire Plc (LON: SHP), the drugmaker that’s made a $30 billion unsolicited offer to buy Baxalta Inc. (NYSE: BXLT), agreed to buy biotechnology company Dyax Corp. (Nasdaq: Dyax) for at least $5.9 billion in cash to gain a promising treatment for a rare genetic disorder.

Shire will pay $37.30 for each Dyax share, the companies said in a statement on Monday. That’s a 35 percent premium over the stock’s Friday closing price. Dyax investors may also get an additional $646 million, or $4 for each share, if a compound known as DX-2930 receives approval for treating hereditary angioedema, or HAE, a disease that can be debilitating and sometimes fatal.

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