Greek concern takes Wall Street down

A weaker-than-expected revenue forecast from Dell also weighs on investor sentiment

U.S. stocks fell at the open on Wednesday on mounting concerns over Greece's future in the euro zone and as a weaker-than-expected revenue forecast from computer maker Dell weighed on investor sentiment.

The Dow Jones industrial average dropped 64.41 points, or 0.52%, to 12,438.40. The S&P 500 Index fell 7.93 points, or 0.60%, to 1,308.70. The Nasdaq Composite Index lost 24.59 points, or 0.87%, to 2,814.49.