Connetics, one of Silicon Valley’s biggest biotechnology companies, will be bought for $640 million by Stiefel Laboratories of Coral Gables, Fla., under a proposed deal announced Monday.

See Mercury News story.

If the sale is approved by shareholders of Connetics, a Palo Alto company that makes skin treatments, the privately held Stiefel would pay $17.50 a share for Connetics stock. That’s a 49 percent premium over the share price of $11.71 at the stock market’s close Friday.

“The combination of Stiefel and Connetics will create the world’s premier dermatology company,” according to a statement issued by Thomas Wiggans, Connetics’ chief executive officer. “We look forward to putting together two powerful corporate cultures.”