Amgen Inc. agreed to buy Onyx Pharmaceuticals Inc. in a $10.4 billion transaction that gives Amgen access to a rapidly expanding cancer market with a new product that offers sure revenue.

Amgen will pay $125 a share for Onyx’s outstanding stock, the companies said in a statement yesterday. Onyx’s Kyprolis, approved last year for a rare blood cancer, may spur more than $3 billion in revenue by 2021, according to analyst estimates compiled by Bloomberg. South San Francisco-based Onyx is now studying the medicine in an expanded group of patients.

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