TOKYO -- The Japanese billionaire behind a deal that would create the world's third-biggest mobile company spent his childhood in a slum, where he proudly rode in a wheelbarrow with pig feed, pushed by his grandmother, a Korean immigrant.

The unlikely success story of Softbank Corp. president Masayoshi Son has taken another leap with his latest megadeal, announced Monday, to take a 70 percent stake in U.S. mobile phone carrier Sprint Nextel Corp. for $20 billion.

The biggest Japanese acquisition of a foreign company underlines Son's unusual status in a corporate culture that has long favored stability over risk-taking. Yet big deals are not the only reason Son has stood out.

He studied in the U.S., graduating from the University of California, Berkeley, and boasts American friends such as Microsoft's Bill Gates. But his high-profile acknowledgment of his Korean roots may be what most sets Son apart in Japan, which has a history of discrimination against its Asian neighbor.

Though initially met with some skepticism from credit rating agencies, Son's foray into the U.S. may inspire similar moves by other Japanese companies as the strength of the yen makes overseas deals more affordable.

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Son, Japan's second-richest man with a fortune of $7.2 billion, according to Forbes, said he identified with the entrepreneurial spirit of older Japanese pioneers such as Soichiro Honda, who started the automaker that carries his name, and Sony Corp. co-founders Akio Morita and Masaru Ibuka.

Son invented a pocket translator while at Berkeley, which was later bought for $1 million by Japan's Sharp Corp. He returned to Japan and used the money to start Softbank in 1981. In its early days, Softbank sold computer software and then went into publishing. Since then it has grown into an empire of Web and mobile businesses.

Son acknowledged he had grown conservative over the past two years as Softbank paid back debt from earlier acquisitions. He didn't like that change in himself. He decided he was ready to take new risks.

"Taking up a challenge always entails a big risk," Son, 55, said at a joint news conference Monday with Sprint's chief executive Dan Hesse.

Son started making headlines in the 1990s with his aggressive acquisitions of companies, which eventually included the Japan units of Yahoo and Vodafone. He became a household name by pushing broadband services more than a decade ago, when the Internet was still relatively new in Japan.