Spark Therapeutics (NASDAQ:ONCE) stock was flying high Monday on news of a deal with Roche.

The deal between Spark Therapeutics and Roche will have the latter buying the gene therapy company for $114.50 per share. This represents a total offering of $4.80 billion for the company when including an extra $500 million in cash at closing.

The offer of $114.50 per share for Spark Therapeutics is a hefty premium to ONCE stock’s closing price on Friday. The stock ‘s closing price at that time was $51.56. This means the offer from Roche is a roughly 122% premium to that price.

Spark Therapeutics says that the deal will have Roche commencing a tender offer to acquire a majority of shares of ONCE stock. Once it has a majority of these shares, it will use a second step merger to acquire the remaining shares. This will be at the same price as in the tender offer.

Spark Therapeutics’ Board of Directors is giving its unanimous support to the offer from Roche. It it advising all ONCE stock holders to offer up their shares during the tender offer period.

So long as everything goes well, Spark Therapeutics and Roche are expecting the deal to close during the second quarter of 2019. This will have ONCE continuing operations in Philadelphia as an independent company.

“With its worldwide reach and extensive resources, Roche will help us accelerate the development of more gene therapies for more patients for more diseases and further expedite our vision of a world where no life is limited by genetic disease,” Jeffrey Marrazzo, CEO of Spark Therapeutics, said in a statement.

ONCE stock was up 120% as of Monday afternoon.

As of this writing, William White did not hold a position in any of the aforementioned securities.