For pure-play coverage of investment-grade mortgage pass-through debt issued by the major US agencies, MBB brings the total package. The fund offers excellent coverage, but to be fair, so do two competing fundsVMBS and SPMBthat track the same or similar indexes. The fund was first to market in this space in 2007, and has a hefty asset base to show for it. MBB is competitively priced after iShares slashed the fund`s fee in July 2017. In all, MBB is a solid choice for MBS exposure.