Simon Property Group is the largest retail real estate investment trust in the United States. Its portfolio includes an interest in 254 properties: 114 traditional malls, 108 premium outlets, 14 Mills centers (a combination of a traditional mall, outlet center, and big-box retailers), six lifestyle centers, and 12 other retail properties. Simon's portfolio averaged $736 in sales per square foot over the trailing 12 months. The company also owns a 22% interest in Klépierre, a European retail company with investments in shopping centers in 14 countries, and joint-venture interests in 33 premium outlets across 14 countries.