Microsoft Corp co-founder Paul Allen has indicated he may sell 1 million shares of the world's largest software giant, a U.S. regulatory filing showed on Thursday. The stock the multibillionaire plans to sell is valued at $64.25 million, according to a filing made with the U.S. Securities and Exchange Commission. The filing, dated Feb. 7 but made public Thursday, does not indicate if the shares have been sold. It reveals, however, the shares were part of founders' stock he acquired in June 1981. Allen's spokeswoman was not immediately available for comment on the filing. The document also shows that Allen has already sold 4.5 million shares for $280.5 million in February. Microsoft stock rose $1-5/16 to $59-11/16 on Nasdaq in late-afternoon trading. In the past 52 weeks, it has been as high as $115 and as low as $40-1/4. Allen, who stopped running the Redmond, Wash.-based company's day-to-day operations in the early 1980s, stepped down from its board of directors last year. He now heads Vulcan Ventures, which he uses for making private investments in a growing number of high-tech and media companies.