Wall St opens flat as Yellen's speech awaited

Wall Street opened little changed on Friday as cautious investors braced for a speech by Federal Reserve Chair Janet Yellen for clues on the timing of a U.S. interest rate hike. Yellen is due to speak at an event hosted by Harvard University at 1:15 p.m. ET (1715 GMT). A rate hike in June was seen as highly unlikely until recently.