U.S. stocks open narrowly mixed ahead of Yellen's testimony

U.S. stocks opened narrowly mixed on Wednesday as investors were awaiting the beginning of Fed Chair Janet Yellen's two-day testimony to Congress.

Yellen will appear before the House Financial Services Committee later Wednesday morning to deliver her testimony on the economy. Investors kept a close eye on the testimony for further indications on the timing of an interest rate hike.

Analysts originally expected interest rates to rise in June, but due to weaker-than-expected employment data, expectations were pushed back to September. Yellen said last Friday that the United States was on track for an increase in the interest rate this year.

On the economic front, the U.S. Producer Price Index (PPI) for final demand advanced 0.4 percent in June, beating market consensus of 0.3, the Department of Labor reported Wednesday.

Overseas, a landmark agreement had been reached over the Iranian nuclear issue between Iran and six world major countries on Tuesday, ending 12 years of negotiations with a win-win agreement.

Under the deal, sanctions imposed by the United States, the European Union and the United Nations will be lifted in return for Iran accepting long-term cuts to its nuclear program.

U.S. stocks posted gains for the fourth straight session on Tuesday, as investors were encouraged by the Iran nuclear deal while digesting economic data and the second-quarter earnings reports from big companies.

Shortly after the opening bell, the Dow Jones Industrial Average fell 19.58 points, or 0.11 percent, to 18,034.00. The S&P 500 lost 1.47 points, or 0.07 percent, to 2,107.48. The Nasdaq Composite Index edged up 2.64 points, or 0.05 percent, to 5,107.53. Endi