(CNBC) The chairman and CEO of Berkshire Hathaway announced today, on his 90th birthday, that his company has acquired a slightly more than 5% stake in each of the five leading Japanese trading companies. The companies are Itochu Corp., Marubeni Corp., Mitsubishi Corp., Mitsui & Co., and Sumitomo Corp. Berkshire said it acquired the holdings over a roughly 12-month period through regular purchases on the Tokyo Stock Exchange. Based on Friday’s closing prices for the trading houses, a 5% stake in each would be valued at roughly $6.25 billion.