Amgen will acquire Horizon Therapeutics, a biopharmaceutical company that focuses on treatments for rare, autoimmune diseases, for about $26.4 billion.

Each Horizon shareholder will receive $116.50 per share for each share they own. The deal has an enterprise value of about $28.3 billion, Amgen said.

Shares of Horizon jumped nearly 14% before the opening bell Monday.

U.S.-traded shares of Horizon soared about 30%, past $100, after the company confirmed late last month that it had begun “highly preliminary discussions” about an acquisition with Amgen, the French drugmaker Sanofi, and Johnson & Johnson’s Janssen division.

Amgen confirmed discussions were taking place in early this month and said at the time that any offer it made for Horizon likely would be in cash, but also that there was no guarantee an offer would be made.

A day later, Johnson & Johnson said that it did not intend to make an offer.

Amgen said Monday that the Horizon transaction will give it access to a complementary portfolio of medicines from Horizon that address the needs of patients suffering from rare diseases.

Horizon Therapeutics PLC, based in Dublin, develops potential treatments for rare, autoimmune and severe inflammatory diseases. Its best-seller, Tepezza, is only approved in the United States and treats eye bulging and double vision from thyroid eye disease.

U.S. regulators approved Tepezza in early 2020 as the first treatment for thyroid eye disease. The drug’s sales more than doubled last year to $1.67 billion. That’s about half of the company’s total sales of $3.23 billion.

The FDA gave Tepezza “orphan drug” status, which comes with financial incentives and exclusive marketing, benefits that are intended to encourage drugmakers to develop rare disease treatments.

Horizon also makes Krystexxa for uncontrolled gout. Sales of that treatment grew 39% last year to $565.5 million.

Shares of Amgen Inc., based northwest of Los Angeles in Thousand Oaks, California, fell nearly 3% in premarket trading.