BioMarin Pharmaceuticals Inc said it would acquire Prosensa Holding NV, a Dutch drug developer for approximately $840 million which includes milestone payments. BioMarin is aiming to add to its portfolio of drugs to treat rare diseases.

This acquisition will give BioMarin worldwide rights to several orphan-drug candidates including drisapersen which is used for treating Duchenne muscular dystrophy (DMD), which is a rare muscle degeneration disease.

For the rare genetic disease, drisapersen could be the first approved treatment. This genetic disease affects one in 3,600 newborn boys; most of them die by age 30.

For each share of Prosensa, BioMarin will offer $17.75 and make two milestone payments of $80 million each following European and U.S. approvals for drisapersen which is Prosensa’s lead drug.

Excluding milestone payments, the offer represents a premium of 55 percent to Prosensa’s last closing price of $11.44. On Monday, shares of Prosensa were trading at $18.50 premarket.

Drugs for rare diseases typically cost more as there are only a few available treatment options and only a handful of companies are involved in developing them.

Prosensa has filed for marketing approval of its lead drug. Rival drug firms Sarepta Therapeutics Inc. and PTC Therapeutics Inc. are also developing treatments for DMD.

Last month Sarepta received a major setback when U.S. regulators asked for more data on eteplirsen, this move could potentially delay the drug further. In 2010, BioMarin dropped its own DMD candidate, after the drug failed early-stage studies.

Both Prosensa and Sarepta use the same mechanism in their Duchenne muscular dystrophy treatments. The drugs skip a faulty section of a gene to produce dystrophin, a protein. The lack of dystrophin causes the disease.

Prosensa is developing two other DMD drugs, which are being tested currently in early and mid-stage studies. The company is also testing drugs to treat Huntington’s disease.

BioMarin said that if drisapersen receives early approvals in the United States and Europe, the acquisition will be accretive to net earnings in 2017.