Jazz Pharmaceuticals said it will buy Gentium for about $1 billion in a deal that will bolster Jazz’ portfolio of rare-disease drugs—notably the acquired company’s lead product Defitelio (defibrotide), the first treatment marketed in Europe for severe hepatic veno-occlusive disease (VOD). The drug won approval just two months ago.

“Because Defitelio is already approved in the European Union, the acquisition would add a new orphan product that has potential for short- and long-term revenue generation, high growth, and expansion of our multinational commercial platform,” Jazz Pharmaceuticals chairman and CEO Bruce Cozadd said in a statement.

“Incorporating Gentium into Jazz Pharmaceuticals is a strong strategic fit as Defitelio would diversify our development and commercial portfolio and complement our clinical experience in hematology/oncology and our expertise in reaching targeted physicians who treat serious medical conditions,” Cozadd added.

Defitelio is indicated for adults and children undergoing hematopoietic stem cell transplantation.

The transaction, expected to close in the first quarter of 2014, has already been approved by the boards of both companies, with Jazz saying it has so far persuaded shareholders, including directors and senior managers, to tender, in combination, roughly 15% of outstanding Gentium ordinary shares and American depositary shares (ADSs).

The deal is subject to customary closing conditions, and a condition that at least 66.67% of Gentium ordinary shares and ADSs be tendered, though Jazz Pharmaceuticals can reduce that percentage to a majority.

At $57 a share, Jazz’ offer represents a 2.4% premium over Gentium’s December 16 closing price for its American depositary receipts.

Last year Jazz added the acute lymphoblastic leukemia drug Erwinaze™ (asparaginase Erwinia chrysanthemi) to its portfolio byacquiring EUSA Pharma for $650 million up front and up to $60 million in payments tied to milestones.