ZymoGenetics, one of Seattle's biggest biotech companies, is being acquired by pharmaceutical giant Bristol-Myers Squibb for approximately $885 million, the companies announced this afternoon.

It's the latest in a series of acquisitions of homegrown Seattle biotech companies by titans of the industry. ZymoGenetics was founded in 1981 by three University of Washington professors.

ZymoGenetics, based in a distinctive converted steam plant on Seattle's Lake Union, employs about 320 people. The companies say no final decisions have been about ZymoGenetics' employees or its properties. However, layoffs are common in such acquisitions and appear inevitable in this situation, as well.

"We don't know who will be laid off and we won't know until the close of the deal," said ZymoGenetics spokeswoman Susan Specht.

The transaction has been unanimously approved by the boards of directors of both companies. Bristol-Meyers Squibb and ZymoGenetics had an existing relationship as collaborators on the development of a Hepatitis C treatment.

In an email to employees, made public in an SEC filing, ZymoGenetics CEO Doug Williams said positive results from the Hepatitis C treatment, known as pegylated-interferon lambda, clearly "had a lot to do" with Bristol Meyers Squibb's interest in acquiring ZymoGenetics.

"However, there is more to it than that," he added. "As a company, ZymoGenetics appealed to Bristol-Myers Squibb as a way to further augment their biologics platform and expand their immunology and oncology pipeline. Our Recothrom business will become part of its U.S. Cardiovascular & Metabolics Business Unit."

Recothrom, the company's first internally developed product, is used to help control bleeding during surgery.

Williams continued, "Overall, the acquisition offers an opportunity for our molecules to progress in Bristol-Myers Squibb’s larger, dedicated infrastructure, as single agents and potentially in combination with their compounds, and with the benefit of their more extensive resources."

Jennifer Fron Mauer, a Bristol-Myers Squibb spokeswoman, said the company is "impressed" by ZymoGenetics' R&D and sales and marketing departments. But she said no decisions will be made on ZymoGenetics staff or real estate until after the acquisition is completed, expected in early October.

A ZymoGenetics spokeswoman, Susan Specht, said Bristol-Myers doesn't have any "near-term" plans to close the Seattle site and plans to keep it open at least through 2011.

The acquisition price equates to $9.75 per share in cash, an 84 percent premium over ZymoGenetics' stock price of $5.30 per share on as of the Tuesday market close. The company's shares skyrocketed to $9.80 in after-hours trading. The deal still requires the approval of ZymoGenetics shareholders, but investors representing 37 percent of the company's shares have agreed to the deal.

ZymoGenetics posted a net loss of $6.2 million for the first six months of this year, on revenue of $20.9 million. The company reported $205.5 million in cash and short-term investments as of June 30. Net of cash to be acquired in the acquisition, the purchase price equals $735 million, the companies said.

Post updated at 3:45 p.m. with more details, context