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Biogen

has agreed to acquire ​Apellis ​Pharmaceuticals

for about $5.6 ⁠billion in cash, expanding its portfolio of rare-disease medicines, the companies ​said on ​Tuesday.

The deal ​marks Biogen’s push to bolster growth as revenue from its multiple sclerosis drugs wanes and ⁠as ‌it seeks to build a broader ⁠rare-disease portfolio, including an expansion into kidney disease treatments.