Stocks were making an effort to extend last week's big rally but struggled to move much higher in midday trading Monday. Existing home sales fell in November to their weakest level in six months. The National Association of Realtors reported sales dropped 6.1%. Gilead Sciences (GILD) is slipping more than 13% after Express Scripts (ESRX) decided to make AbbVie's (ABBV) drug the exclusive option for patients with hepatitis C. Express Scripts manages prescriptions for insurance plans and is up better than 1%. Cubist Pharmaceuticals (CBST) is rising after the FDA's approval of its new antibiotic to fight intra-abdominal and urinary tract infections. Shares of American Apparel (AAP) are up and finally trading above a dollar after the retailer confirmed it is considering a buyout offer.