Vertex Pharmaceuticals Inc. said Tuesday it plans to pay $1.1 billion to Crispr Therapeutics AG as part of a deal to test an experimental gene-editing therapy for sickle cell disease and transfusion-dependent beta-thalassemia. The companies previously set up a development deal in 2015; as part of the amended one, which was announced Tuesday, Vertex will pay the Swiss genome-editing company a new $900 million upfront payment, with an additional $200 million available for meeting regulatory milestones, to test the investigational therapy for the blood disorders. In exchange, Vertex will provide 60% of costs and get 60% of profits from future sales if the therapy is approved and commercialized. Since the start of the year, Vertex’s stock is down 6.8%, shares of Crispr Therapeutics have dropped 20.5%, and the S&P 500 is up 10.1%.

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