MBB's largest holdings are BlackRock Funds III: BlackRock Cash Funds: Institutional; SL Agency Shares, Government National Mortgage A 5.5 09/15/2037, Freddie Mac, FNMA TBA 5.5% SEP 30YR, FNMA TBA 5.0% SEP 30YR. The full top list is on this page.

How concentrated is MBB?

The top 10 holdings are 14.4% of the fund, across 11,222 positions.

The total expense ratio on record is 0.04%. That is cheaper than 94% of the 864 US-listed bond funds we track.

How similar is MBB to the US aggregate index (AGG)?

19.2% of MBB overlaps with the US aggregate index (AGG) by weight, across 1,114 shared companies.

US-listed · United States

MBB

seeks to track the investment results of an index composed of investment-grade, mortgage-backed pass-through securities issued and/or guaranteed by U.S. government agencies.

iShares MBS ETF

US4642885887 · IRUK · BlackRock (iShares)

11,222 holdings as of 2026-08-27 · top 10 = 14.4% · TER 0.04%

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Concentration

Well diversifiedtop 10 = 14.4%

diversifiedconcentrated

GROWTH · PRICE DEVELOPMENTrebased to 0%

Holdings

11,222

as of 2026-08-27

Top 10

14.4%

TER

0.04%

cheaper than 94% of peers

Net assets

US$39.7B

reported in USD

Largest

7.6%

P / E

–

Sections

Holdings as of 2026-08-27.

01Overview

the fund's regulatory facts

Fund currency

USD

Domicile

United States

First listed

2007-02-28

Volatility 1Y

4.7%

Tracked index

Bloomberg U.S. MBS Index

How it tracks

Index, sampling

02Performance

market price return · as of 2026-09-11

-1.9%

-3.2%

-3.7%

YTD

-4.0%

-4.3%

Trackingfund minus Bloomberg U.S. MBS Index · annualised

1Y return gap

-0.06 pp/yr

3Y return gap

-0.01 pp/yr

5Y return gap

-0.04 pp/yr

3Y tracking error

0.17%

A positive return gap means the fund finished ahead of the index over that window. The 3Y tracking error shows how variable that monthly gap was; lower means steadier tracking. MethodologyReturn gap and tracking errorReturn gap is the fund's return minus its index over the same months, converted to a yearly rate. Tracking error is the annualised standard deviation of those monthly fund-minus-index returns. Both lines come from the manager's own chart, in the same currency and the exact index variant the fund tracks. We use month-end values so different intraday valuation times are not mistaken for tracking error.. Not investment advice.

03Holdings

50 largest · as of 2026-08-27

#	Company	Identifier	Weight
1	BlackRock Funds III: BlackRock Cash Funds: Institutional; SL Agency Shares US	BISXX	7.9
2	Government National Mortgage A 5.5 09/15/2037 US	G2SF	1.1
3	Freddie Mac US	FRSD8146	1.0
4	FNMA TBA 5.5% SEP 30YR US	FNCL	1.0
5	FNMA TBA 5.0% SEP 30YR US	FNCL	0.7
6	Government National Mortgage A 5 09/15/2037 US	G2SF	0.6
7	Government National Mortgage Association US	US3618N5A332	0.5
8	Government National Mortgage Association US	US36179WP937	0.5
9	Government National Mortgage Association US	US36179WTX64	0.5
10	Fannie Mae US	US31418D7E66	0.5
11	GNMA II TBA 6.0% SEP 30YR JMBO US	US21H0606978	0.5
12	Government National Mortgage Association US	G2	0.5
13	Freddie Mac US	US3132DWBH51	0.5
14	G2SF 4.50 09/26 TBA US	G2SF	0.5
15	Fannie Mae US	FNMA4465	0.4
16	Fannie Mae US	US3140XLE460	0.4
17	GNMA II POOL MB1070 US	US3618N6FL65	0.4
18	UMBS 15YR TBA(REG B) US	UM15	0.4
19	Government National Mortgage Association US	G2MA9171	0.4
20	Government National Mortgage Association US	US36179WY855	0.4
21	Government National Mortgage Association US	US36179WFG87	0.3
22	FNMA TBA 6.0% SEP 30YR US	UM30	0.3
23	Fannie Mae US	US3140XREC56	0.3
24	Fannie Mae US	US31418DXK35	0.3
25	Government National Mortgage Association US	G2MA7826	0.3
26	Fannie Mae or Freddie Mac 2 12/31/2049 US	UM15	0.3
27	GNMA II POOL MB1142 US	US3618N6HU47	0.3
28	Fannie Mae US	FNMA4437	0.3
29	Government National Mortgage Association US	G2MA7534	0.3
30	Fannie Mae US	FNMA4305	0.3
31	Fannie Mae US	US3140QHBC84	0.3
32	Government National Mortgage Association US	US36179WY939	0.3
33	Government National Mortgage Association US	US36179VZQ66	0.3
34	Government National Mortgage Association US	US36179WTY48	0.3
35	GNMA2 30YR US	US3618N6P684	0.3
36	Government National Mortgage Association US	US36179Y2N36	0.3
37	Freddie Mac 5 04/01/2056 US	US3142J6DQ38	0.2
38	Freddie Mac US	US3132DWG982	0.2
39	Freddie Mac US	US3132DWCT80	0.2
40	Government National Mortgage Association US	US3618N5C569	0.2
41	FREDDIE MAC POOL FR 05/53 FIXED 4 US	FR	0.2
42	Fannie Mae Pool US	FN	0.2
43	Government National Mortgage Association US	G2MA7987	0.2
44	Government National Mortgage Association US	G2MA7589	0.2
45	Freddie Mac US	US3132DWBB81	0.2
46	Freddie Mac US	US3132DWJF14	0.2
47	Fannie Mae US	US3140XHHS99	0.2
48	Freddie Mac US	US3133KLYA96	0.2
49	Fannie Mae US	US3140QHKW48	0.2
50	Fannie Mae US	US3140QVG328	0.2

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04Exposure

Bonds 92.3%Cash 7.4%

By country

USUnited States101.6%

CurrencyUSD 101.6%

05Similar funds

funds with comparable exposure

AGG

the US aggregate index (AGG)

1,114 shared companies

19.2%compare →

Ranked by how much of their holdings you already own through this fund. Overlap by weight where shown.

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