U.S. home builders gain after Berkshire Hathway reveals stake

** Shares of home builders Lennar and D.R. Horton rise premarket after Warren Buffett's Berkshire Hathway reveals stake

** Lennar LEN gains 3.5%, D.R. Horton DHI adds 2.8%

** Berkshire reveals stakes of 1.5 million shares in DHI, and an increased stake in LEN

** Buffett's conglomerate said it was buying the two co's before April, but SEC let it keep purchases confidential so investors wouldn't piggyback

** Stock prices often rise after Berkshire's disclosures, with investors viewing them as Buffett's seal of approval

** As of last close, LEN up 0.8% and DHI up 18.3% YTD compared to S&P 1500 Homebuilding index's .SPCOMHOME gain of 10.6%