Roche is to buy U.S.-based Spark Therapeutics for $4.3 billion and give the Swiss drugmaker a foothold in gene therapy.

Roche will pay $114.50 per share for Spark more than twice the loss-making U.S. company's closing price on February 22nd.

Basel-based Roche is buying Spark and its portfolio of treatments for blindness, haemophilia and neurodegenerative diseases as rivals including Novartis also move aggressively into gene therapy.

Roche - the biggest cancer drugs maker - was late to the game in immuno-oncology Analysts say the Spark deal will give Roche a platform for getting gene therapies to market, but comes with risks... including getting beaten by rivals.

Novartis has made gene therapy one of its focus areas giving it a head start on Roche.

Investors appeared a little unsure of the news too Roche shares were trading 0.8 percent lower in Monday morning trade.