Sanofi SA will buy Blueprint Medicines Corp. for $9.1 billion equity value, in a deal that sees the European pharmaceutical firm expand its rare immunological disease portfolio.

Shares of Blueprint Medicines Corp. surged 26% in premarket trading on the news.

Sanofi will pay $129 per share in cash, the companies said in a statement Monday. That represents a 27% premium to Blueprint’s closing price on Friday.

Blueprint shareholders will also receive one non-tradeable contingent value right, which will pay the holder $2 and $4 per right for the achievement, respectively, of future development and regulatory milestones for BLU-808 — a potential treatment for patients with mast cell disorders, including chronic urticaria.

The deal comes as Sanofi has touted its ambition to become an immunology powerhouse, and earlier this year announced a pact to buy an autoimmune drug from Dren Bio, Inc. for up to $1.9 billion.

“Sanofi still retains a sizeable capacity for further acquisitions,” Chief Executive Officer Paul Hudson said in the company’s statement.

Blueprint currently markets a medicine for a rare condition known as systemic mastocytosis, where an abnormal buildup of immune cells can cause symptoms such as hives, abdominal pain, bone pain and anemia. The drug, known as Ayvakit in the US and Ayvakyt in the Europe, is projected to bring in $700 million to $720 million in revenue in 2025, per company guidance.

Sanofi said Blueprint also brings a promising pipeline of unapproved immunology drugs and an “established presence” among specialist doctors.

The acquisition marks the culmination of a successful pivot by Blueprint, which was founded in 2008 and listed in 2015 with a focus on cancer therapies. After the dissolution of a partnership with Roche Holding AG forced it to withdraw an approved cancer from certain markets, it dialed back its efforts in cancer and doubled down on immunology.

Including the potential CVR payments, the deal amounts to about $9.5 billion on a fully diluted basis.

Sanofi expects to complete the acquisition in the third quarter and said the deal won’t have a significant impact on its financial guidance for 2025.