U.S. stocks kicked off Tuesday's session slightly lower, as investors anxiously awaited a key speech from Federal Reserve Chair Janet Yellen. Experts expect Yellen to provide clues on the timing of the next rate hike. Scott Wren, senior global equity strategist at Wells Fargo Investment Institute expects some type of language in the speech about when the central bank will hike rates next. He also expects one rate hike in 2016. Meanwhile, Berkshire Hathaway (BRK.A) - Get Report CEO Warren Buffett increased his stake in Wells Fargo (WFC) - Get Report to 506 million shares, or just under 10 percent of the bank's outstanding shares. This may trigger increased Fed scrutiny so regulators can keep an eye on who the bank's biggest owners are. Finally, Yahoo! (YHOO) told potential buyers of its Web business that they have until April 11 to submit a bid, according to reports from the Wall Street Journal. Verizon Communications (VZ) - Get Report and Time Inc. (TIME) are among the long list of companies that are reportedly interested in Yahoo!. TheStreet's Scott Gamm has details from Wall Street.