Simon Property Group is the largest retail real estate investment trust in the United States. Its portfolio includes an interest in 232 properties: 134 traditional malls, 73 premium outlets, 14 Mills centers (a combination of a traditional mall, outlet center, and big-box retailers), six lifestyle centers, and five other retail properties. Simon's portfolio averaged $733 in sales per square foot over the trailing 12 months. The company also owns a 22% interest in Klép