MBSD tracks a proprietarily weighted index of MBSs to achieve and maintain an effective duration of 3.25-4.25 years. Unlike other conventional market-value-weighted broad MBS indexes with slow-moving durations, the fund provides predictability in interest-rate risk exposure. With explicit and implicit government guarantee, agency-backed MBSs carry negligible credit risk but higher yields due to prepayment risk. In combination, MBSD offers investors a unique exposure to generate higher income than Treasurys and with predictable interest-rate risk. Rebalancing is done on a monthly basis.