To pursue its goal, the fund generally invests in securities that are included in the Bloomberg US MBS Float Adjusted Total Return Index. The index tracks fixed-rate agency mortgage-backed pass-through securities guaranteed by the Government National Mortgage Association (Ginnie Mae), the Federal National Mortgage Association (Fannie Mae), and the Federal Home Loan Mortgage Corporation (Freddie Mac) that are backed by pools of mortgages. The index is constructed by grouping individual to-be-announced (TBA) deliverable mortgage-backed securities (MBS) pools into aggregates or generics based on program, coupon, and vintage. To be included in the index, pool aggregates must have at least dollar 1 billion currently outstanding and a weighted average maturity of at least 1 year. The index excludes certain types of securities, including among other things, TBAs, collateralized mortgage obligations (CMOs), non-agency (whole loan) collateral, Fannie Mae mega pools, Freddie Mac giant pools, Freddie Mac super pools, Ginnie Mae.