Mortgage-Backed Security (MBS) Instrument and Market Features

Fixed Income, LOS weight share 1.1 percent of the 365 Level I learning outcomes.

Fixed IncomeMortgage-Backed Security (MBS) Instrument and Market Features

Rates falling is good news for almost every bond an investor holds, except the mortgage-backed one, where falling rates trigger a wave of refinancing that hands back principal exactly when reinvesting it becomes least attractive.

Before you watch

Answer these three first. Getting them wrong now is normal, and it helps the lesson stick. Reveal the answers when you're done, then read on.

1. Interest rates fall sharply and mortgage prepayments accelerate across a pass-through MBS pool. This exposes the investor primarily to:

Answer: B. Falling rates make refinancing attractive to homeowners, accelerating prepayments and shortening the pool's effective life, contraction risk, which forces the investor to reinvest the returned principal at the now-lower prevailing rates. Extension risk is the opposite scenario, triggered by rising rates.

2. A collateralized mortgage obligation (CMO) is best described as a structure that:

Answer: B. A CMO reallocates total prepayment risk among tranches with different priority rules, directing more of the variability to support (companion) tranches and less to PAC tranches; the total prepayment risk in the structure still equals the risk in the underlying pool, it has been redistributed, not eliminated.

3. A commercial mortgage-backed security (CMBS) differs from a residential MBS primarily because CMBS:

Answer: A. CMBS are backed by loans on income-producing commercial real estate (office, retail, multifamily, industrial) rather than owner-occupied homes, and commercial mortgage loans typically include structural call-protection features (prepayment lockouts, yield maintenance, or defeasance) that limit the borrower's ability to prepay freely, unlike the largely unrestricted prepayment option on most residential mortgages.

The lesson

There is no video lesson for this unit yet. Everything this unit teaches is written out below.

The reading

Five to ten minutes on this one unit: what the exam wants, the idea in plain words, then straight into the trap and the practice.

The exam wants you to describe the features of residential mortgages that shape MBS cash flows, describe contraction and extension risk, describe how a CMO's tranching redistributes prepayment risk across a PAC and a support tranche, and compare agency MBS, non-agency MBS and CMBS.

A residential mortgage loan is typically long-dated and fully amortizing. It is secured by a lien on the property. It usually carries an unrestricted prepayment option too: the borrower can pay down principal early, in full or in part, at no penalty. That prepayment option is the structural source of every prepayment risk passed through to an MBS investor. It produces two directionally opposite risks the exam expects you to tell apart on sight.

Falling rates trigger contraction risk. Homeowners rush to refinance. Prepayments accelerate. The security's effective life shortens. The investor gets principal back sooner than expected, right when reinvesting it at the new, lower rates is least attractive. This runs backward from ordinary bond intuition, where falling rates are simply good news for a holder. For an MBS, falling rates instead trigger the prepayment option against the investor. Rising rates trigger the mirror image, extension risk. Homeowners have no reason to refinance. Prepayments slow or stop. The security's effective life stretches out beyond what was expected, and the investor is stuck holding a now-below-market coupon for far longer than planned. Both risks genuinely harm the investor. They simply arrive from opposite rate directions. This asymmetry is exactly why a mortgage pass-through security exhibits negative convexity. When rates fall, the price gain a straight bond would post is capped, because accelerating prepayments hand back the high-coupon asset just as its price would otherwise be rising.

A collateralized mortgage obligation restructures a mortgage pool's cash flows into tranches with different priority rules. It redistributes prepayment risk; it never eliminates it. The total prepayment risk in the pool stays exactly conserved across every tranche combined. A planned amortization class tranche is designed to receive a stable, scheduled principal repayment, as long as actual prepayments stay inside a specified band. That stability is funded by pushing the variability that would otherwise disrupt the schedule onto a support, or companion, tranche instead. A support tranche absorbs that pushed-off variability and is compensated with a higher yield for doing so. A mortgage pass-through, by contrast, gives every investor a pro-rata share of the pool's cash flows and its prepayment risk in identical proportion. The CMO reallocates that same aggregate risk unevenly instead of spreading it evenly.

Agency MBS carry government backing, but not all of the same kind, and the exam tests the distinction precisely. GNMA, a literal US government agency, carries an explicit full faith and credit guarantee from the Treasury. FNMA and FHLMC are government-sponsored enterprises instead: publicly chartered but privately owned. Their backing is implicit only, a market expectation rather than a legal guarantee. The 2008 crisis made that distinction real when both were placed into government conservatorship. Non-agency MBS carry no government backing at all and rely on credit enhancement instead. Commercial mortgage-backed securities pool loans secured by income-producing property: office, retail, industrial, hotel or multifamily. Repayment there depends on the property's own operating income, not a single homeowner's paycheck. Commercial borrowers tend to be larger and more sophisticated, and lenders want more certainty over the loan's life. That is why CMBS carry structural protections, lockout periods, yield maintenance, defeasance, that residential mortgages generally lack. It is exactly why CMBS carry comparatively little of the prepayment risk that dominates residential MBS.

The trap

Any answer choice claiming a CMO eliminates prepayment risk is automatically wrong: a CMO only redistributes the pool's total prepayment risk across its tranches, funding a PAC tranche's stability by pushing more of that same risk onto the support tranche, never by destroying any of it.

Learning outcomes covered by this module

Verbatim from the 2026 CFA Level I topic outline. Every practice question and key rule below is tagged to one of these where the stem and explanation make the match clear.

  1. define prepayment risk and describe time tranching structures in securitizations and their purpose
  2. describe fundamental features of residential mortgage loans that are securitized
  3. describe types and characteristics of residential mortgage-backed securities, including mortgage pass-through securities and collateralized mortgage obligations, and explain the cash flows and risks for each type
  4. describe characteristics and risks of commercial mortgage-backed securities

Key rules

Written from this module's own lesson and the 2026 CFA Level I topic outline, in teaching order, each tagged to the learning outcome it belongs to where that is clear.

LOS 01

Prepayment risk on a mortgage pass-through has two directionally opposite components

Contraction risk arises when interest rates fall: homeowners refinance, prepayments accelerate, the security's effective life shortens, and the investor must reinvest the returned principal at the now-lower rates. Extension risk arises when interest rates rise: homeowners have no incentive to refinance, prepayments slow, the security's effective life lengthens beyond what was expected, and the investor is stuck holding a below-market coupon for longer than planned. Neither risk is universally bad, each simply describes what happens when the rate environment moves against what the investor wanted; both stem from the same underlying prepayment option held by the borrower, not the investor.

LOS 01

A CMO uses tranching to redistribute, not eliminate, the prepayment risk of its underlying pool

Time tranching splits a mortgage pool's cash flows into separate classes with different priority rules for receiving principal; a planned amortization class (PAC) tranche is designed to receive a stable, scheduled principal repayment as long as actual prepayments stay within a specified band, while a support (companion) tranche absorbs the prepayment variability that would otherwise disrupt the PAC's schedule, receiving less principal when prepayments are slow and more when they are fast. Because the total prepayment risk in the pool is conserved, the PAC tranche's added stability is purchased at the cost of greater risk, and a correspondingly higher yield, borne by the support tranche.

LOS 02

The residential mortgages underlying an RMBS carry features that directly shape the security's cash flows

A residential mortgage loan is typically a long-dated, fully amortizing loan secured by a lien on the property, and it typically carries an unrestricted prepayment option, the borrower may pay down principal early, in full or in part, without penalty, which is the structural source of the prepayment risk passed through to MBS investors. Loan features such as the interest rate type (fixed versus adjustable), the term, and whether the loan is a conforming loan (meeting size and underwriting standards for agency purchase) all affect the prepayment and credit behavior of the pool built from those loans.

LOS 03

Pass-through securities and CMOs represent the same underlying cash flows structured two different ways, each with a different risk profile

A mortgage pass-through security gives every investor a pro-rata share of the pool's principal and interest cash flows exactly as they come in, with each investor bearing prepayment risk in identical proportion. A CMO restructures those same aggregate cash flows into multiple tranches with different, non-pro-rata priority rules, allowing investors to select a risk and cash-flow profile (stable PAC versus volatile support) that a plain pass-through cannot offer. Both structures are exposed to the same total prepayment risk originating in the underlying loans; the pass-through simply does not redistribute it.

LOS 04

CMBS are secured by income-producing commercial property and are structured to limit the prepayment risk that dominates residential MBS

A commercial mortgage-backed security pools loans secured by income-producing commercial real estate, office, retail, industrial, hotel, or multifamily properties, where repayment depends on the property's ability to generate rental or operating income rather than a single homeowner's paycheck. Because commercial borrowers are typically large, sophisticated entities and lenders want more certainty over the loan's cash flow stream, commercial mortgage loans commonly include structural protections against early prepayment, such as prepayment lockout periods, yield maintenance provisions, or defeasance requirements, which sharply reduce the contraction risk that is central to residential MBS. CMBS credit risk instead centers on property-level and tenant-level performance, and CMBS structures typically use credit tranching by seniority rather than the prepayment-focused time tranching central to residential CMOs.

The trick

Authored only where a key rule has an arbitrary number, list, or formula shape worth a memory device; a module with none of those has no tricks here, on purpose.

Falling rates = contraction (shorter life, reinvest lower). Rising rates = extension (longer life, stuck low)

The direction is the opposite of ordinary bond-pricing intuition; for a straight bond, falling rates are simply good news, for an MBS, falling rates trigger the prepayment option against the investor.

CMOs redistribute prepayment risk; the word 'eliminates' in an answer choice about CMOs is always wrong

Total pool prepayment risk is conserved across all the tranches combined; a PAC tranche's stability is funded by pushing more variability onto the support tranche, not by destroying risk.

CMBS prepayment protection comes from loan-level structural features, not the absence of a prepayment option

Lockout periods, yield maintenance, and defeasance are the mechanisms that suppress contraction risk in commercial mortgages; residential mortgages generally lack these restrictions, which is exactly why residential MBS carry substantial prepayment risk and CMBS carry comparatively little.

The method

Authored, ordered steps for answering this module's question types; a calculation module's calculator-dependent step ends with a bracketed BA II Plus keystroke sequence.

  1. For a rate-direction question, map falling rates to contraction risk (shorter life, reinvest at lower rates) and rising rates to extension risk (longer life, stuck with a low coupon).
  2. For a CMO-purpose question, answer 'redistributes' prepayment risk, never 'eliminates' it, and identify whether the tranche in question is a PAC (protected, lower yield) or support (absorbs variability, higher yield).
  3. For a pass-through versus CMO question, remember both carry the same total pool risk; the pass-through spreads it pro-rata while the CMO reallocates it unevenly across tranches.
  4. For a CMBS question, look for structural prepayment protection (lockout, yield maintenance, defeasance) and property/tenant-level credit risk as the distinguishing features from residential MBS.

One card

Condensed from the key rules and tricks above, nothing new. What you'd want on one index card the night before.

Practice questions

Pick an answer, say how sure you are, then reveal. Every wrong choice gets its own explanation. 10 question(s) available for this unit.

Question 1Exam level

When mortgage interest rates fall sharply, an investor holding a mortgage pass-through security is most likely exposed to:

How sure are you?

Correct: B. When rates fall, homeowners have an incentive to refinance their mortgages, which accelerates principal prepayments to the pass-through's investors. This is contraction risk: the security's average life shortens and the investor must reinvest the returned principal at the new, lower prevailing rates.
A. Extension risk is the opposite scenario: it occurs when rates RISE and homeowners have no incentive to refinance, so prepayments slow and the security's duration stretches out longer than expected. Falling rates produce the reverse effect, contraction, not extension.
C. A mortgage pass-through's cash flows are explicitly NOT fixed like a standard bond's: because the underlying mortgages can be prepaid at any time, the investor's actual principal and interest cash flows depend on the pace of prepayments, which is exactly the risk being tested here.

Unit: mortgage-backed-security-mbs-instrument-and-market-features

Question 2Exam level

In a sequential-pay collateralized mortgage obligation (CMO) with tranches A, B, and C, the tranche most likely to bear the greatest extension risk is:

How sure are you?

Correct: B. In a sequential-pay structure, principal is distributed to tranche A first, then B, then C, in strict order. Tranche C waits longest to begin receiving principal, so if prepayments slow (rates rise), tranche C's average life stretches out the most of the three. Tranche A, receiving principal first, is the most exposed to contraction risk instead.
A. Tranche A is the one MOST exposed to contraction risk (prepayments arriving faster than expected), not extension risk, since it is first in line for whatever principal comes in. The question asks about extension risk, which lands hardest on the last tranche, not the first.
C. A CMO's tranching structure redistributes prepayment risk unevenly among the tranches; it never eliminates the risk itself. 'CMOs eliminate prepayment risk' is one of the most common traps on this topic; the correct concept is redistribution, not elimination.

Unit: mortgage-backed-security-mbs-instrument-and-market-features

Question 3Exam level

Within a single CMO structure, compared to a support tranche, a PAC (planned amortization class) tranche most likely offers:

How sure are you?

Correct: B. A PAC tranche is structured to receive a predictable schedule of principal payments as long as actual prepayments stay within a specified band (the PSA range the structure was designed around); the support tranche absorbs the prepayment variability that keeps the PAC's cash flows stable. Investors accept a lower yield on the PAC in exchange for that predictability.
A. A higher yield for greater variability describes the SUPPORT tranche, not the PAC. The PAC is the more stable, lower-yielding tranche precisely because the support tranche is absorbing the prepayment risk on its behalf.
C. Receiving no principal until every other tranche is retired describes a sequential-pay tranche's position in the payment queue, not what distinguishes a PAC from a support tranche. A PAC tranche does receive a defined principal schedule of its own throughout the structure's life.

Unit: mortgage-backed-security-mbs-instrument-and-market-features

Question 4Exam level

Compared to a mortgage-backed security guaranteed by GNMA (Ginnie Mae), one guaranteed by FNMA (Fannie Mae) most likely carries:

How sure are you?

Correct: B. GNMA is a U.S. government agency, so its guarantee carries the full faith and credit of the U.S. government, an explicit guarantee. FNMA and FHLMC are government-sponsored enterprises (GSEs), privately owned companies with a government charter; their guarantee is implicit, not backed directly by the government's full faith and credit.
A. GNMA is the one with the explicit, full-faith-and-credit government guarantee, because it is literally a federal government agency. FNMA is a GSE, a separate legal category with only an implicit guarantee, so the two are not identical on this point.
C. An FNMA MBS does carry credit support, just of the implicit, GSE-backed kind rather than no support at all; the distinction tested here is explicit versus implicit guarantee, not guarantee versus no guarantee.

Unit: mortgage-backed-security-mbs-instrument-and-market-features

Question 5Exam level

The PSA prepayment benchmark's standard ramp assumes the conditional prepayment rate (CPR) most likely:

How sure are you?

Correct: A. The 100% PSA benchmark ramps the CPR up from 0.2% in the pool's first month, increasing by a fixed increment each month, until it reaches 6% in month 30, then holds at 6% for the remaining life of the pool. A 200% PSA assumption runs at twice that speed at every point on the ramp.
B. A constant 6% CPR from month 1 ignores the ramp-up period entirely; the PSA benchmark's defining feature is that prepayment speed starts low and builds up over the pool's first 30 months before leveling off, not a flat rate from day one.
C. This reverses the direction of the ramp. The PSA benchmark models prepayment speeds INCREASING over the pool's early life (as more homeowners become eligible or motivated to prepay), not decreasing.

Unit: mortgage-backed-security-mbs-instrument-and-market-features

Question 6Exam level

A mortgage pass-through security's price appreciation when interest rates fall is most likely limited by:

How sure are you?

Correct: A. A pass-through MBS behaves like a bond with an embedded call option (the homeowners' prepayment option). As rates fall, prepayments accelerate, returning principal at par just when the investor would otherwise want to hold a high-coupon bond whose price is rising, capping further price appreciation. This is negative convexity, the opposite of a straight bond's positive convexity.
B. Positive convexity describes a straight, non-callable bond's price behavior, where price gains accelerate as rates fall. A pass-through MBS displays the opposite pattern, negative convexity, precisely because of the prepayment option embedded in the underlying mortgages.
C. There is no statutory price ceiling set at origination. The price-limiting effect comes from the economics of prepayment (the embedded call option), not from any regulatory or contractual cap on the security's price.

Unit: mortgage-backed-security-mbs-instrument-and-market-features

Question 7Exam level

The primary purpose of transferring mortgage loans into a bankruptcy-remote special purpose entity (SPE) during securitization is most likely to:

How sure are you?

Correct: A. The SPE is structured to be bankruptcy-remote: the transferred mortgage loans are legally isolated from the originator, so if the originator later becomes insolvent, its creditors cannot reach the collateral backing the securities already sold to investors. That legal isolation is the core purpose of the SPE step in securitization.
B. The whole point of the SPE transfer is that the originator no longer legally owns the loans; ownership moves to the SPE. The originator may still be retained as loan servicer for a fee, but legal ownership is exactly what is transferred away.
C. An SPE isolates the collateral from the originator's OTHER creditors; it does not eliminate the credit risk inherent in the mortgage pool itself. Investors in the securities still bear the risk that the underlying homeowners default.

Unit: mortgage-backed-security-mbs-instrument-and-market-features

Question 8Exam level

Compared to residential mortgage-backed securities, commercial mortgage-backed securities (CMBS) are most likely structured with:

How sure are you?

Correct: A. Because commercial mortgage borrowers are typically large, sophisticated entities, CMBS pools are commonly structured with call protection at both the loan level and the structure level, such as a lockout period during which prepayment is prohibited, defeasance (the borrower substitutes government securities as replacement collateral), or yield maintenance charges that compensate investors for lost interest on a prepayment. This makes CMBS cash flows more predictable than residential MBS.
B. CMBS, like residential CMOs, are commonly tranched by seniority and credit risk, with junior/subordinate tranches absorbing losses before senior tranches; pooled pro rata loss sharing with no tranching at all is not the standard CMBS structure.
C. CMBS prepayment behavior is deliberately made LESS like residential MBS prepayment through the call protection features described above; individual commercial borrowers do not prepay the way a large, diversified pool of individual homeowners does, and the structure is built specifically to blunt that difference in behavior, not replicate it.

Unit: mortgage-backed-security-mbs-instrument-and-market-features

Question 9Above the exam

An investor holds a CMO support tranche during a period when interest rates fall sharply, causing prepayments to run far above the PSA band the structure was designed around. Combining the role of a support tranche with the PAC tranche's protection band, this investor should most likely expect:

How sure are you?

Correct: B. The support tranche exists specifically to absorb the prepayment variability that keeps the PAC tranche's cash flows stable within its designed band. When prepayments run far above that band (as they would with a sharp rate decline), the support tranche absorbs the EXCESS principal being returned, shortening its own average life dramatically (contraction risk) precisely so the PAC tranche can continue to receive its planned, stable schedule. This is the defining trade-off of the PAC/support structure.
A. CMO tranching exists precisely because different tranches do NOT receive identical protection; the PAC tranche is structured for stability specifically because the support tranche absorbs the variability instead, which is the whole mechanism this question tests.
C. A support tranche's entire purpose is to be LESS stable than the PAC tranche, not functionally identical to it; describing them as equivalent misses the core PAC/support risk-redistribution relationship.

Unit: mortgage-backed-security-mbs-instrument-and-market-features

Question 10Above the exam

An investor is choosing between a GNMA-guaranteed pass-through and a similar-coupon, similar-maturity non-agency (private-label) pass-through with no credit enhancement beyond the quality of the underlying loans. Combining the guarantee structure of each with the concept of required yield compensation for risk, the non-agency security should most likely:

How sure are you?

Correct: B. GNMA carries the explicit, full-faith-and-credit guarantee of the US government, eliminating credit risk for the investor. A non-agency security with no credit enhancement bears the underlying loans' actual credit risk directly; investors require additional yield compensation for taking on that risk that the GNMA security's investors do not bear at all. All else equal (coupon, maturity), the non-agency security must offer a HIGHER yield to attract investors given its greater credit risk.
A. Competitive pricing among issuers does not eliminate the fundamental credit risk difference between a government-guaranteed security and one with no credit enhancement; investors still require compensation for that real, un-guaranteed credit risk regardless of how issuers price against each other.
C. Being 'mortgage-backed' in both cases does not make the two securities equivalent in risk; the guarantee structure (explicit government guarantee vs. none at all) is exactly the differentiating factor that drives a required yield difference between them.

Unit: mortgage-backed-security-mbs-instrument-and-market-features

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