UPDATE, 5/21/13, 6:40 p.m. EDT: Latham & Watkins is providing Yahoo with regulatory advice with respect to the Tumblr acquisition. Latham’s team is led by antitrust and competition partners Hanno Kaiser, in San Francisco; Amanda Reeves, in Washington, D.C.; and Susanne Zuehlke, in Brussels.

Simpson Thacher & Bartlett is advising Yahoo on the onetime Internet leader’s largest acquisition in a decade: the $1.1 billion purchase of New York–based blogging and social media platform Tumblr.

The all-cash deal—news of which began to leak over the weekend ahead of a statement confirming it Monday—is the first $1 billion-plus acquisition that Sunnyvale, California–based Yahoo has made since paying $1.6 billion to buy search company Overture Services in 2003, according to The Wall Street Journal. The Journal adds that Marissa Mayer, the former Google executive hired as Yahoo’s CEO last summer, described the Tumblr transaction as an "exception" to her company’s previously stated strategy of growing through smaller acquisitions geared more toward adding talent than technology.

In one such deal, Yahoo paid a reported $30 million in March to acquire mobile news aggregation reader Summly, the creation of 17-year-old British app entrepreneur Nick D’Aloisio. Weil, Gotshal & Manges provided IP counsel to the company on that deal, according to our prior reporting.

Vowing "not to screw [Tumblr] up" in a statement announcing the transaction—which is expected to close in the second half of the year—Yahoo said the newly acquired company will be operated independently and that founder David Karp will remain as CEO. Tumblr has more than 300 million unique visitors each month, while 120,000 new users join the service each day, according to the Yahoo announcement.

VentureBeat notes that the challenge so far for Tumblr, which took in just $13 million in revenue last year, has been making money. And while the deal won’t solve that problem right away, Mayer said it will increase Yahoo’s audience by 50 percent and its traffic by roughly 20 percent.

The Tumblr acquisition is the latest major tech industry transaction in which Simpson has had a hand. The firm is also representing investment firm Silver Lake in connection with its joint bid, with Michael Dell, to buy computer maker Dell Inc. for $24.4 billion. In 2008 the firm actually represented Microsoft Corp. in an attempted bid to take over Yahoo. (Simpson was eventually conflicted out of that representation, though, only to be replaced by Sullivan & Cromwell.)

The Simpson Thacher team advising Yahoo on the Tumblr deal is led by New York–based M&A partner Gary Horowitz. In Palo Alto, M&A partner Kirsten Jensen is also advising along with tax partner Katharine Moir and compensation and benefits partner Tristan Brown. The Simpson associates working on the matter are Linda Barrett, Sean Crnkovich, Andrew Nightingale, and Cara Walsh.

Yahoo’s general counsel is Ron Bell, a former attorney at Dentons predecessor firm Sonnenschein Nath & Rosenthal who succeeded onetime Skadden, Arps, Slate, Meagher & Flom associate Michael Callahan when the latter left Yahoo last summer. The company has relied on Skadden for counsel frequently on various transactions in the past, including last year’s $7.1 billion sale of a 20 percent stake in Chinese Internet company Alibaba.

Tumblr, meanwhile, has turned to a team from technology-focused shop Gunderson Dettmer Stough Villeneuve Franklin & Hachigian, which has served as the company’s outside general counsel since its 2007 founding. New York–based corporate partners Ward Breeze, Steve Baglio, and Dan Goldberg are leading the firm’s team on the deal.

Ari Shahdadi, a former Gunderson attorney, serves as Tumblr’s general counsel.