Latham & Watkins was the year's most active U.S. law firm advising on transactions valued under $1 billion. The Los Angeles firm's 420 M&A attorneys closed 86 middle-market deals, totaling $26.5 billion in value-a significant amount more than the firm's 2012 tally of 76, worth $19.7 billion .

One of the largest law firms in the country, Latham boasts clients among large public companies, such as Apple Inc. (Nasdaq: AAPL) and Yahoo Inc. (Nasdaq: YHOO). But the firm's middle-market practice is "huge," says Howard Sobel (pictured), a partner in the New York office. Typically, the firm advises on deals that fall between $250 million and $1 billion. For example, Latham partner Tad Freese piloted Yahoo's purchase of Summly for a reported $30 million.

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