USG Corp. said Monday it has rejected the unsolicited buyout bid by Gerb. Knauf AG, saying it “substantially undervalues” the company and is not in the best interest of its shareholders. Gebr. Knauf has bid $42 a share in cash, which is 25% above Friday’s closing price of $33.51, and would give USG a market capitalization of $5.92 billion. “Our Board is always looking for ways to deliver value to all of our shareholders, but Knauf’s opportunistically timed proposal is wholly inadequate as it does not reflect USG’s intrinsic value, including the significant opportunities ahead of us,” said USG Non-Executive Chairman Steven Leer. Earlier, Warren Buffett’s Berkshire Hathaway Inc. , USG’s largest shareholder with a 30.8% stake, said it offered Gebr. Knauf an option to buy its USG stake. USG’s stock soared 19% in premarket trade. It has tumbled 12.6% over the past three months through Friday, while the S&P 500 has slipped 3.4%.