Mission West Properties, Inc. acquires, markets, leases, and manages research and development (R&D) properties, primarily located in the Silicon Valley portion of the San Francisco Bay Area. As of December 31, 2008, the Company owned and managed 111 properties totaling approximately 8 million rentable square feet of R&D properties through four limited partnerships, or operating partnerships, for which the Company are the sole general partner. R&D property is designed for research and development and office uses and, in some cases, includes space for light manufacturing operations with loading docks. There are two tenants who individually lease in excess of 300,000 rentable square feet from the Company: Microsoft Corporation and Apple Computer, Inc. The Company operates as a Real Estate Investment Trust (REIT). As of December 31, 2008, the Company acquired 23 leased R&D properties totaling approximately 2,243,000 rentable square feet.