The fund invests at least 80% of its total assets in Government National Mortgage Association mortgage-backed securities, which are debt securities representing part ownership in a pool of mortgage loans backed by the full faith and credit of the U.S. government. It invests the balance of its assets primarily in other "U.S. government Obligations," which are defined as U.S. Treasury bills, notes and bonds, and other bonds and mortgage-backed securities issued or guaranteed by the U.S. government, or by government sponsored enterprises.