A host of Am Law firms have secured advisory roles in connection with British pharmaceutical giant GlaxoSmithKline’s unsolicited bid to purchase biotechnology outfit Human Genome Sciences for $2.59 billion.

London-based GSK confirmed Thursday that it has offered to acquire Human Genome for $13 in cash per share. GSK chief executive Sir Andrew Witty formally contacted Human Genome CEO Thomas Watkins on April 11 in a letter outlining the offer.

Rockville, Maryland–based Human Genome, meanwhile, released a statement Thursday in which the company’s board claimed that the GSK offer—which represents an 81 percent premium over the target company’s Wednesday closing price—“does not reflect the value inherent” in Human Genome. While adding that it is exploring alternatives to the GSK offer, the board left the door open to a deal eventually getting done.

The companies have a history of collaboration, including the joint development of multiple drugs now in the Human Genome pipeline. Those products include drugs aimed at treating heart disease and diabetes, as well as Benlysta—a lupus treatment that received FDA approval last year.

The New York Times notes that GSK’s bid is likely an attempt to capitalize on a significant drop in Human Genome’s stock price, which has fallen nearly 75 percent in the past year after approaching $30 per share. The company has reported combined losses of more than $600 million in the past two years as a result of marketing and administrative costs related to Benlysta.

GSK said in its announcement that it expects a deal would generate cost savings of at least $200 million by 2015, and that the proposed deal would add to earnings by next year.

In his letter to Watkins, GSK CEO Witty revealed that his company has retained Cleary Gottlieb Steen & Hamilton and Wachtell, Lipton, Rosen & Katz as legal counsel on its unsolicited bid.

The New York–based Cleary team is being led by M&A partner Victor Lewkow, corporate finance partner Benet O’Reilly, and litigation partner Meredith Kotler. GSK is a longtime Cleary client, with the firm having represented SmithKline Beecham in its 2000 merger with Glaxo Wellcome—a deal valued at roughly $76 billion.

For Wachtell, the representation signifies a bit of a switch, as the firm—whose founding partner Martin Lipton invented the “poison pill” takeover defense—is frequently tapped by target companies looking to fend off takeover bids. New York–based corporate partners Adam Emmerich and David Lam are leading the Wachtell team working with GSK.

Gibson, Dunn & Crutcher corporate partners Dennis Friedman and Eduardo Gallardo are representing Lazard and Morgan Stanley in their roles as financial advisers to GSK.

Human Genome has engaged Skadden, Arps, Slate, Meagher & Flom and DLA Piper as outside counsel in the process.

Skadden’s team is being led by Washington, D.C.-based M&A partners Marc Gerber and Michael Rogan. A DLA Piper spokesman did not immediately respond to a request for further information about its role in the matter.

Fried, Frank, Harris, Shriver & Jacobson corporate partners Philip Richter and Robert McLaughlin are advising Goldman Sachs in its role as a financial adviser to Human Genome.