It is a mortgage-backed securities bond ETF that tracks the Bloomberg US MBS Index. It holds a broad range of investment-grade, fixed-rate pass-through securities issued or guaranteed by US government agencies such as Ginnie Mae, Fannie Mae and Freddie Mac.

Backed by agency guarantees, it suits core bond investors seeking stable interest income and diversification.

It is a passively managed (index-tracking) ETF run by iShares.

The fund holds fixed-rate mortgage-backed securities issued and guaranteed by US government agencies in line with index weights, tracking the performance of the Bloomberg US MBS Index. It uses a sampling approach together with to-be-announced (TBA) mortgage transactions to replicate the index's economic characteristics, and credit risk is relatively low thanks to the agency guarantees.

• Concentration in agency-guaranteed mortgage securities
• Low expense ratio and large asset base

Assets under management (AUM) stand at $35.8B, and the expense ratio is 0.04% per year.

Fees are comparable to those of similar ETFs such as VMBS and SPMB, while asset size and liquidity differ.

Mortgage-backed security prices react sensitively to the direction of market interest rates and to the prepayment environment. Price volatility appears during periods of changing rates, and overall return trends tend to depend on the bond yield environment.

Fund flows tend to be steady, supported by stable demand for agency-guaranteed bonds. Bond ETFs tend to attract more inflows when expectations of rate cuts grow, while rising rates can weigh on prices.

Its strengths are low credit risk from agency guarantees and a low expense ratio, while its weakness is sensitivity to interest rates and prepayments.

The two ETFs shown in the table, VMBS and SPMB, are direct substitutes in the same agency mortgage-backed securities category, and their low fee levels make them worth reviewing together from a cost perspective. Meanwhile, JMBS and SMBS differ in issuer and strategy approach, leading to differences in fees and management style, so they are worth comparing. For broader bond exposure, AGG, which tracks the US aggregate bond market, and SHY, which focuses on short-term Treasuries, can also be considered for diversification.

Here are the points to review before investing in MBB. Because these are agency-guaranteed bonds, credit risk is relatively low, but it is wise to examine the interest rate direction, the prepayment environment, and the exchange rate effect on won-converted returns before buying.

Rising rates and prepayments are the main drivers of short-term volatility. Credit risk is relatively low thanks to the agency guarantees, but price swings tied to the interest rate environment are hard to avoid.

It is a suitable choice for core bond investors looking to diversify into agency-guaranteed mortgage bonds. If you weigh costs and asset size together, comparing it with VMBS and SPMB is recommended.