iShares MBS ETF (NASDAQ:MBB) Holds 10,821 Agency Mortgage-Backed Securities: Structure, TBAs and Yield

Sonal Goyal

27 October 2026 05:28 PM AEDT

iShares MBS ETF (NASDAQ:MBB) Holds 10,821 Agency Mortgage-Backed Securities: Structure, TBAs and Yield

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Key Highlights

iShares MBS ETF (NASDAQ:MBB) had net assets of about $35.60 billion and a NAV of $91.45 as of Oct 26, 2026.
The fund tracks the Bloomberg U.S. MBS Index with an expense ratio of 0.04%.
Its 30-day SEC yield was 4.43%, with monthly distributions.

The iShares MBS ETF (NASDAQ:MBB) tracks U.S. agency mortgage-backed securities. As of Oct 26, 2026, it held net assets of about $35.60 billion and a NAV of $91.45 per share.

Index and Investment Approach

The fund follows the Bloomberg U.S. MBS Index and has traded since Mar 13, 2007. It charges 0.04%. Most transactions in fixed-rate mortgage pass-through securities occur through to-be-announced (TBA) transactions, which the fund regularly uses.

Portfolio Composition

The fund held 10,821 securities. Excluding a BlackRock cash fund (4.47%), the largest holdings were an FHLMC 30-year UMBS Super (1.05%), a UMBS 30-year TBA (0.60%) and GNMA2 30-year securities (0.59% and 0.54%). Effective duration was 5.67 years.

Prepayment and Extension Risk

iShares notes that mortgage-backed securities are subject to prepayment and extension risk and react differently to interest rate changes than other bonds. The fund invests cash pending TBA settlement in money market instruments and similar short-term instruments.

Yield and Distributions

The fund reported a 30-day SEC yield of 4.43% as of Oct 23, 2026 and a 12-month trailing yield of 4.46%. Distributions are paid monthly. The most recent distribution was $0.366185 per share, with an ex-date of Oct 1, 2026 and payment on Oct 6, 2026. iShares notes that distribution yield and trailing yield can vary from period to period for reasons such as tax treatment and changes in fund shares outstanding.

Portfolio Characteristics

As of Oct 23, 2026 the portfolio held 10,821 securities. Weighted average coupon was 3.64, weighted average maturity 7.58 years, effective duration 5.67 years and average yield to maturity 5.64%, with an option adjusted spread of 34.01 bps and convexity of -0.09.

Concentration and Holdings Profile

10,821 securities were held as of Oct 23, 2026. The five largest positions listed on the fund page totalled about 7.25% of assets, leaving roughly 92.75% across the remaining holdings. That is a low level of concentration, with exposure spread thinly across many issuers. iShares notes that holdings are subject to change and that the weights shown are calculated from vendor prices, which may differ from the fund’s own valuation prices.

Fund Details at a Glance

The iShares MBS ETF (NASDAQ:MBB) is an index-based fixed income fund that began trading on Mar 13, 2007. Its benchmark is the Bloomberg U.S. MBS Index, and it pays distributions monthly. As of Oct 26, 2026 it had 389.3 million shares outstanding and net assets of $35,600,187,780. An annual fee of 0.04% works out to $4 for every $10,000 held, before any brokerage commissions.

Risk Disclosures Noted by iShares

Fixed income risks include interest-rate and credit risk: when interest rates rise, the value of debt securities typically declines, and credit risk is the possibility that an issuer cannot make principal and interest payments. Mortgage-backed securities are subject to prepayment and extension risk, and small movements in interest rates may quickly and significantly reduce their value. Shares of ETFs are bought and sold at market price rather than NAV and are not individually redeemed from the fund, and brokerage commissions will reduce returns. Investing involves risk, including possible loss of principal.

Costs and Trading Details

The expense ratio is 0.04%. The fund is listed on Nasdaq with 389.3 million shares outstanding and net assets of $35,600,187,780 as of Oct 26, 2026. The closing price was $91.29 as of Oct 23, 2026, against a NAV of $91.45 on Oct 26, and the premium/discount was 0.01. The 30-day median bid/ask spread was 0.01% and 30-day average volume was 6,663,008 shares.

Closing View

iShares MBS ETF (NASDAQ:MBB) reported net assets of about $35.60 billion and 10,821 holdings, with its index, fees and distribution schedule as set out above. The monthly distribution schedule, 0.04% expense ratio and 0.01% median bid/ask spread are the main cost and income details reported for the fund.

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