Wall Street lower after CPI data; Yellen speech awaited

Yellen is expected to highlight the economy's steady job growth, while April's consumer price index data is likely to bolster Fed's case for its first policy tightening in nearly a decade. Yellen's speech will start at 1:00 p.m. ET (1700 GMT). Consumer prices moderated last month, data showed, but the so-called core consumer price index, which strips out food and energy costs, posted its largest gain since January 2013.