Strategy. Under normal circumstances, the Fund seeks to achieve its objective by investing at least 80% of its net assets, plus any borrowings for investment purposes, in mortgage-backed and other mortgage-related securities that are issued or guaranteed by the U.S. Government and its agencies. The securities in which the Fund may invest include U.S. government securities, U.S. government agency securities, securities issued by U.S. government instrumentalities and U.S. government-sponsored enterprises, and other mortgage-backed securities or mortgage-related securities issued by the U.S. government or by private issuers. Some of these securities are issued and/or guaranteed by the U.S. government and are supported by the full faith and credit of the United States. Other securities are issued or guaranteed by Federal agencies or … Show more… Under normal circumstances, the Fund seeks to achieve its objective by investing at least 80% of its net assets, plus any borrowings for investment purposes, in mortgage-backed and other mortgage-related securities that are issued or guaranteed by the U.S. Government and its agencies. The securities in which the Fund may invest include U.S. government securities, U.S. government agency securities, securities issued by U.S. government instrumentalities and U.S. government-sponsored enterprises, and other mortgage-backed securities or mortgage-related securities issued by the U.S. government or by private issuers. Some of these securities are issued and/or guaranteed by the U.S. government and are supported by the full faith and credit of the United States. Other securities are issued or guaranteed by Federal agencies or government-sponsored enterprises and are not direct obligations of the United States, and are not backed by the full faith and credit of the United States, but involve sponsorship or guarantees by government agencies or enterprises. The Fund invests primarily in the highest rated government and agency bonds and maintains an average portfolio duration that is within 1 year of the duration of the Bloomberg U.S. MBS Index (the Funds benchmark). The Fund will seek to invest in mortgage-backed and other mortgage-related securities that are issued or guaranteed by the U.S. Government and its agencies that BlackRock believes have the potential to produce attractive long-term returns and that, in BlackRocks view, can provide positive and measurable impacts. BlackRock considers a number of criteria when selecting portfolio securities, including, but not limited to, whether the activities supported by the investment are expected to include positive social and/or environmental impact with measurable and clear benefit to undercapitalized or high social opportunity areas and alignment with broadly endorsed public policy goals. Fund management uses an internal proprietary framework to identify investments that deliver positive social and/or environmental impact. This internal proprietary framework abides by the Operating Principles for Impact Management, which is a third-party framework for investors for the design and implementation of their impact management systems to ensure that impact considerations are integrated throughout the investment lifecycle. In identifying investments, Fund management uses internal research and assessment to identify mortgage-related investments that meet one or more of the following criteria: (i) facilitate equal access to credit; (ii) target historically underserved populations; and/or (iii) support the increase of sustainable housing units, which encompasses both affordability and environmental aspects. The Funds investments will be focused across mortgage impact themes that align with certain United Nations Sustainable Development Goals (SDGs), including, but not limited to, ending poverty, promoting inclusive and sustainable industrialization, reducing inequalities, and making cities and communities sustainable. The SDGs are a series of goals published by the United Nations that recognize that development must balance social, economic and environmental sustainability. The Fund intends to focus its investments in mortgages originated as part of social impact programs, including, without limitation, rural housing, manufactured housing, pools issued by State Housing Finance Authorities and bespoke impact mortgage-backed security pools. Additionally, BlackRock may consider specific criteria such as collateral targeted towards underserved populations and/or underserved regions as having a positive social impact. BlackRock will examine both quantitative metrics and qualitative details to measure the impact achieved. The impact-related criteria described above are not the sole considerations when making investment decisions for the Fund, and the Fund will make investments, including, but not limited to, to be announced (TBA) transactions, that do not meet the impact-related criteria. In addition, the Fund may gain indirect exposure (through, including but not limited to, derivatives and investments in other investment companies) to issuers with exposures that are inconsistent with the impact-related criteria used by Fund management. Securities purchased by the Fund (except with respect to non-dollar denominated bonds) generally are rated in the highest rating category (AAA or Aaa) by at least one major rating agency or are determined by the Fund management team to be of similar quality at the time of purchase. The Fund may also invest up to 5% of its assets in dollar-denominated investment grade securities that are rated below the highest rating category at the time of purchase. In addition, the Funds dollar-weighted average maturity will be between 3 and 10 years. The Fund evaluates sectors of the bond market and individual securities within these sectors. The Fund selects bonds from several sectors including: U.S. Treasuries and agency securities (including U.S. Treasury Inflation-Protected Securities), commercial and residential mortgage-backed securities, collateralized mortgage obligations (CMOs), and other mortgage-related securities. The Fund invests primarily in dollar-denominated bonds, but may invest up to 10% of its assets in non-dollar denominated bonds of issuers located outside of the United States. The Funds investment in non-dollar denominated bonds may be on a currency hedged basis. Non-dollar denominated bonds purchased by the Fund, at the time of purchase, are rated in the four highest rating categories by at least one major rating agency (Baa or better by Moodys Investors Service, Inc. (Moodys) or BBB or better by S&P Global Ratings (S&P) or Fitch Ratings Inc. (Fitch)) or are determined by the Fund management team to be of similar quality. Securities rated in any of the four highest rating categories or determined by the Fund management team to be of similar quality at the time of purchase are known as investment grade securities. The Fund may buy or sell options or futures on a security or an index of securities, or enter into credit default swaps, interest rate or foreign currency transactions, including swaps, and forward contracts or similar instruments, such as TBA transactions or dollar rolls (collectively, commonly known as derivatives). The Fund may seek to obtain market exposure to the securities in which it primarily invests by entering into a series of purchase and sale contracts or by using other investment techniques such as reverse repurchase agreements. The Fund may engage in active and frequent trading of portfolio securities to achieve its primary investment strategies. Show less