Under normal market conditions, the Fund will invest at least 80 percent of its net assets (including investment borrowings) in a portfolio of investment-grade debt securities issued or guaranteed by the U.S. government, its agencies or government-sponsored entities, including publicly-issued U.S. Treasury securities and mortgage-related securities. The Fund may also invest in exchange-traded funds (ETFs) that principally invest in such securities. As discussed in more detail below, the Fund may purchase mortgage-related securities in to-be-announced transactions (TBA Transactions), including mortgage dollar rolls. The Fund includes cash set aside or otherwise held as collateral for settling mortgage dollar rolls and other TBA Transactions towards its 80 percent investment requirement. The Funds investment advisor seeks to manage the Funds portfolio to have a weighted average effective duration of eight or more years. Duration, which is discussed in more detail below, measures a debt securitys expected life on a present value basis, taking into account the debt securitys yield, interest payments and final maturity.