U.S. S&P 500 stock futures erased most of their gains on Wednesday after DuPont, a Dow component, cut its fourth-quarter profit outlook. A reported downgrade to "neutral" from a "buy" by Merrill Lynch on Internet media company Yahoo Inc.'s pushed Nasdaq 100 stock futures into negative territory. In Wednesday's pre-open activity, Standard & Poor's 500 index futures were up 0.7 points, but still above fair value, a mathematical formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Nasdaq 100 index futures were down 1 point, and Dow Jones industrial average futures rose 5 points.