U.S. stocks slipped after a disappointing quarterly performance from Alcoa (AA - Get Report) soured optimism over earnings season. The aluminum producer missed on the top- and bottom-line. Yum! Brands (YUM - Get Report) moved higher after detailing plans to drive growth once it separates from its China business. The owner of KFC and Taco Bell said its resulting business will operate as a "pure play" franchisor. Illumina (ILMN - Get Report) slumped after reducing its third-quarter revenue forecast to below its previous guidance range. The biotech company also warned that fourth quarter sales would come in flat to up slightly. Twitter (TWTR - Get Report) was upgraded to HOLD from SELL at Evercore. Analysts said the move was tied to a recent selloff. Shares have been under pressure as a number of potential bidders reportedly backed out of negotiations. TheStreet's Keris Lahiff reports from Wall Street.