The investment seeks investment results that correspond generally to the price and yield performance, before fees and expenses, of the ICE BofA Merrill Lynch® Constrained Duration US Mortgage Backed Securities IndexSM. The underlying index reflects the performance of a selection of investment-grade U.S. agency residential mortgage backed pass-through securities. The fund generally will invest under normal circumstances at least 80% of its total assets (exclusive of collateral held from securities lending) in the securities of the underlying index and in TBA Transactions that represent securities in the underlying index.