Shire (SHPG) - Get Report is buying Viropharma Pharmaceuticals (VPHM) for $4.2 billion to bolster its portfolio of drugs treating rare diseases, the companies announced Monday.

The all-cash deal values Viropharma at $50 per share, or a 27% premium to the company's closing stock price Friday. 

Viropharma's best-selling product is Cinryze, which treats hereditary angioedema, a rare blood disease that causes swelling. Viropharma sales totaled $428 million last year, the bulk coming from Cinryze. The company is expected to generated $445-$465 million in sales this year. 

"By joining with Shire, ViroPharma will become part of a larger, more diverse biopharmaceutical company and will benefit from Shire's innovation, scale and global reach," said Viropharma CEO Vincent Milano, in a statement.

Shire intends to fold Viropharma into its business unit that sells drugs for rare diseases. These include Finryze, which also targets hereditary angioedema, as well as drugs for Gaucher and Fabry disease. 

Once the Viropharma acquisition closes, Shire expects its rare disease unit to generate $2 billion in revenue in 2014, or 40% of its total revenue on a pro forma basis.

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