The investment seeks investment results that correspond generally to the price and yield performance, before fees and expenses, of the Barclays U.S. MBS Index. The fund invests at least 90% of its assets in the securities of the underlying index and in investments that provide substantially similar exposure to securities in the underlying index. The underlying index measures the performance of investment-grade mortgage-backed pass-through securities issued by the Federal National Mortgage Association ("Fannie Mae"), Federal Home Loan Mortgage Corporation ("Freddie Mac") and Government National Mortgage Association ("Ginnie Mae").