Private equity firm Sycamore Partners on Thursday said it was interested in acquiring apparel retailer Express Inc, after disclosing a 9.9 percent stake that made it the largest shareholder in the company. Express confirmed on Thursday in a press statement that it had received a letter from Sycamore, highlighting their interest in buying the company. Columbus, Ohio-based Express has adopted a stockholder rights plan, setting the trigger at 10 percent. Express's shares rose as much as 32.8 percent to $18.00 in extended trading. The stock closed down nearly 3 percent at $13.55 in regular trading on Thursday, valuing the company at $1.14 billion. "We would like to perform confirmatory due diligence to determine a definitive valuation of the company," Sycamore said in a letter to the company's board. ( link.reuters.com/huv99v ) The private equity firm said it would take 30 days to complete its due diligence, obtain financing commitments and submit its offer, accordi