The fund seeks to track the investment returns of its benchmark index, the Bloomberg U.S. Long Treasury Bond Index. It invests at least 80% of its net assets in securities that are held in its benchmark index and at least 80% of its net assets in U.S. Treasury securities, which are backed by the full faith and credit of the U.S. government. The index is an index consisting of U.S. dollar-denominated, fixed rate nominal debt issued by the U.S. Treasury with maturities of 10 years or more.