Spark Therapeutics Inc. (ONCE) - Get Report shares more than doubled Monday after the gene therapy specialist agreed to a $4.3 billion takeover by Switzerland-based Roche Holding  (RHHBY) , the world's biggest maker of cancer treatments.

Roche said it will pay $114.50 per share for Spark, a 122% premium to the stock's Friday close price that would value the Philadelphia, Pa.-based biotech and its portfolio of neurodegenerative disease treatments -- including Luxturna -- at around $4.3 billion. 

"As the only biotechnology company that has successfully commercialized a gene therapy for a genetic disease in the U.S., we have built unmatched competencies in the discovery, development and delivery of genetic medicines. But the needs of patients and families living with genetic diseases are immediate and vast," said CEO Jeffrey Marrazzo. "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."

Spark shares were rising 120.1% to $113.47 Monday. Roche shares drifted modestly lower, down 0.4%.

"Spark Therapeutics' proven expertise in the entire gene therapy value chain may offer important new opportunities for the treatment of serious diseases. In particular, Spark Therapeutics' haemophilia A programme could become a new therapeutic option for people living with this disease," said Roche CEO Severin Schwan. "We are also excited to continue the investments in Spark Therapeutics' broad product portfolio and commitment to Philadelphia as a center of excellence."

Roche also made specific mention of Spark's SPK-8011, a late-stage gene therapy for the treatment of haemophilia A patients, that could help the combined group close the gap crated by last year's $8.7 billion takeover of Bannockburn, Illinois-based Avexis, a specialist in spinal muscular atrophy and is in clinical trials for the first-ever one-time gene replacement therapy for the mostly fatal childhood disease, by Swiss rival Novartis AG (NVS) - Get Report .