A spinal muscular atrophy carrier test estimates whether a person has a nonworking copy of SMN1, the main gene responsible for 5q spinal muscular atrophy (SMA). Unlike many carrier screens that simply sequence a gene, standard SMA testing first counts copies of SMN1 exon 7. Most carriers have one working SMN1 copy instead of the usual two. However, some people have two copies on one chromosome and none on the other, creating a “2+0” silent-carrier result that ordinary copy-number testing can miss.
The report therefore needs more than a positive or negative label. It should state the SMN1 copy number, whether a linked duplication marker such as c.*3+80T>G was tested, the person’s residual carrier risk, and whether family history changes the interpretation. SMN2 copy number is important for an affected person’s broad disease severity and treatment evaluation, but it is not the central measure of routine carrier status. A normal carrier screen reduces risk; it does not eliminate it because silent carriers and rare SMN1 sequence variants exist.
- One copy of SMN1 exon 7 usually means the person is an SMA carrier.
- Two copies reduce carrier risk but can hide a 2+0 silent-carrier configuration.
- A linked duplication marker increases the chance that two copies sit on the same chromosome.
- Three or more SMN1 copies usually lower risk further but do not prove how copies are arranged.
- A family-specific SMN1 sequence variant may require targeted sequencing, not copy-number testing alone.
- When both partners are carriers, each pregnancy has a 25% chance of SMA.
Table of Contents
- Why SMN1 copy number is unusual
- Who should have SMA carrier screening
- How to read one, two, or three SMN1 copies
- Silent carriers and duplication markers
- Family history, partner testing, and residual risk
- Pregnancy and prenatal testing
- Carrier testing versus diagnostic SMA testing
- Limitations and next steps
Why SMN1 copy number is unusual
The survival motor neuron region on chromosome 5 contains two highly similar genes, SMN1 and SMN2. SMN1 normally produces most of the full-length survival motor neuron protein needed by motor neurons. SMN2 differs by a small number of sequence changes that alter splicing, so most SMN2 transcripts skip exon 7 and produce less stable protein. It still provides some full-length protein and can partially compensate when SMN1 is absent.
Most people have one SMN1 copy on each chromosome 5, commonly described as a 1+1 arrangement. Most SMA carriers have one chromosome with one SMN1 copy and the other with zero, a 1+0 arrangement. Most people with 5q SMA have no functional SMN1 copy because both inherited copies are deleted or disrupted.
The complication is that SMN1 copy number varies. A person can have two copies on one chromosome and none on the other, a 2+0 arrangement. A test that counts a total of two copies cannot always distinguish 1+1 from 2+0. That is why a “two-copy” result is a reduced-risk result rather than an absolute negative.
SMN2 copy number is a separate measurement. In people with SMA, more SMN2 copies are often associated with a milder average phenotype because more full-length protein can be produced. The relationship is not exact: people with the same SMN2 count can differ in onset and severity, and disease-modifying variants within SMN2 can affect prediction.
For a person undergoing carrier screening, SMN2 count does not tell whether they are an SMN1 carrier. Some laboratories report SMN2 incidentally or as part of the same assay, but the reproductive interpretation remains centered on SMN1.
A good report names the method—such as multiplex ligation-dependent probe amplification, quantitative PCR, digital PCR, or validated sequencing-based copy-number analysis—and states which exons or markers were measured. It should not imply that “two genes present” equals zero risk.
Who should have SMA carrier screening
SMA carrier screening is commonly offered before pregnancy or during pregnancy regardless of race or ethnicity. Population-neutral offering is important because carriers occur in every ancestry group and appearance cannot establish genetic risk.
Screening is particularly important for:
- A person planning pregnancy who has not previously been screened
- The reproductive partner of a known SMA carrier
- A relative of someone with SMA or a documented SMN1 pathogenic variant
- A couple using donor eggs or sperm when carrier information is needed
- A person whose prior report gave only a vague “negative” result without copy number
- A family in which an earlier pregnancy or child was affected
Preconception testing provides the most time for partner testing, counseling, and consideration of reproductive options. Testing during pregnancy is still useful, but laboratories and clinicians should coordinate timelines so that partner and fetal testing can be completed when needed.
Testing the person with the clearest family diagnosis first is especially valuable. If an affected relative has a common homozygous SMN1 exon 7 deletion, copy-number testing may be informative for relatives. If the relative has one deletion and one rare SMN1 sequence variant, family members need testing that can detect that exact sequence change. A standard dosage assay may miss it.
A prior negative expanded carrier screen should be reviewed rather than automatically repeated. Ask whether the test measured SMN1 copy number, assessed a silent-carrier marker, provided residual risk, and included sequence-variant analysis. Older tests may have reported only exon 7 dosage.
Carrier testing is voluntary. Counseling should explain that the test estimates reproductive risk, does not measure personal muscle strength, and may not provide a yes-or-no answer. A result showing one SMN1 copy is usually straightforward; a two-copy result with a duplication marker is probabilistic.
People who are adopted, donor-conceived, or have limited family history can still be screened. Lack of a known affected relative does not meaningfully reduce population carrier risk enough to skip an offered test.
How to read one, two, or three SMN1 copies
The central line of an SMA carrier report is often the number of SMN1 exon 7 copies. Each count has a different interpretation.
Zero SMN1 copies
Zero copies of SMN1 exon 7 is strongly associated with SMA, not ordinary carrier status. An apparently healthy adult with this result needs urgent laboratory confirmation and clinical review because sample error, unusual gene configurations, or a presymptomatic/mild phenotype may be possible. In a newborn or symptomatic child, zero copies supports the diagnosis and should trigger rapid neuromuscular evaluation.
One SMN1 copy
One copy usually means carrier status. The person likely has a 1+0 arrangement. Carriers are not expected to develop 5q SMA because one functioning copy generally provides enough protein.
Rarely, a person with one detected copy may have a pathogenic sequence variant in that remaining copy and therefore have SMA. Symptoms, family history, and diagnostic testing determine whether that possibility needs investigation. In routine asymptomatic reproductive screening, one copy is reported as carrier positive.
Two SMN1 copies
Two copies is the most common result. It usually reflects one copy on each chromosome, but the assay may not distinguish 1+1 from 2+0. Residual carrier risk remains because of silent carriers and rare sequence variants.
The report may refine that risk using a linked duplication marker. A marker-positive result raises the chance of a 2+0 arrangement. A marker-negative result lowers it but does not exclude it. Risk estimates differ by ancestry because the association between the marker and copy arrangement differs across populations.
Three SMN1 copies
Three copies often reflect a 2+1 arrangement and generally makes a 3+0 silent-carrier arrangement unlikely. Residual risk is lower than with two copies but not zero. The test still may not detect a rare pathogenic sequence variant.
Four or more copies
Higher copy numbers are uncommon and usually further reduce carrier probability. Copy number alone cannot always show exact chromosome arrangement, and unusual family histories still require targeted analysis.
Reports should avoid calling every two-copy or three-copy result simply “negative.” Better wording is “reduced carrier risk,” followed by an estimate and test limitations. A carrier-screening result guide can help place residual risk in context.
Silent carriers and duplication markers
A silent carrier has two SMN1 copies on one chromosome and no SMN1 on the other. The total copy number is two, so basic dosage testing appears normal even though the person can pass the zero-copy chromosome to a child.
Certain variants are associated with SMN1 duplication alleles. The most commonly reported marker is c.3+80T>G, also known in older literature as g.27134T>G. Some laboratories also assess c.211_*212del. These markers are not disease-causing by themselves. They serve as clues that two SMN1 copies may be located together.
A two-copy, marker-positive result means increased silent-carrier risk, not confirmed carrier status. The same marker can occur in people whose two copies are arranged 1+1, and not every 2+0 carrier has the marker. The report should provide an ancestry-informed or pan-ethnic residual-risk estimate rather than a binary label.
A two-copy, marker-negative result reduces the chance of a 2+0 configuration but does not remove it. It also does not address carriers who have one normal-copy count plus a rare pathogenic sequence variant.
Family studies can sometimes resolve the arrangement. If parents or children are tested, inheritance of copy numbers and markers may show whether two copies travel together on one chromosome. This analysis is most useful in a family with an affected child or a known carrier, where the stakes and prior probability are higher.
Ancestry categories used in risk tables are imperfect. People may have mixed ancestry, may not know their background, or may not fit the study populations used to calculate estimates. Laboratories should explain the assumptions and avoid treating race as a biological yes-or-no category. The marker result is molecular evidence; ancestry only modifies the statistical estimate.
Silent-carrier analysis is a good example of why “screen negative” does not mean “no chance.” The practical question is whether the remaining risk is low enough for the couple or whether family history, partner status, or personal preferences justify further testing.
Family history, partner testing, and residual risk
Residual risk is the chance that a person is still a carrier after a reduced-risk result. It depends on the starting carrier frequency, test detection rate, SMN1 copy number, duplication-marker status, ancestry data, and family history.
A general population two-copy result may reduce risk substantially. The same result in the sibling of a known carrier does not carry the same reassurance because the prior risk is higher and the family may have a specific 2+0 allele or sequence variant. Laboratories’ generic tables should not replace pedigree-based calculation.
When one partner has one SMN1 copy, the other partner should have carrier testing. If the second partner has two copies, the couple’s risk becomes their residual carrier risk multiplied by the chance of an affected child if both are carriers. A genetics professional can translate that into a pregnancy-specific estimate.
If both partners are confirmed carriers, each pregnancy has:
- A 25% chance of a child with SMA
- A 50% chance of a carrier child
- A 25% chance of a child who inherited neither familial pathogenic allele
These percentages assume each parent contributes one nonworking SMN1 allele and follow autosomal recessive inheritance. Rare new deletions or complex gene conversions can alter family interpretation, especially when an affected child’s parents do not both appear to be carriers.
Approximately a small minority of SMA cases involve a new deletion in one parental contribution rather than two inherited carrier alleles. When a child has SMA but only one parent tests as a carrier, the laboratory and genetics team should review parentage, sample identity, de novo events, mosaicism, and assay limitations before giving recurrence counseling.
Cascade testing should use the affected person’s or carrier relative’s actual report. If the family variant is a point mutation, testing only SMN1 dosage can falsely reassure relatives. If the family has a documented 2+0 allele, marker and phase analysis may be more important than a population residual-risk table.
A result from direct-to-consumer raw data is not adequate for this purpose. The SMN region is highly homologous and copy-number interpretation requires a validated clinical assay.
Pregnancy and prenatal testing
When both partners have significant carrier risk, reproductive choices depend on timing, values, and the exact familial findings.
Before pregnancy, options may include natural conception with or without prenatal diagnosis, in vitro fertilization with preimplantation genetic testing for a monogenic condition, donor egg or sperm, adoption, or choosing not to have children. During pregnancy, diagnostic testing can be performed on chorionic villus or amniotic-fluid samples when the familial variants or copy-number changes are defined.
Prenatal diagnosis should be designed by a laboratory experienced with the SMN1/SMN2 region. Testing may need to distinguish SMN1 from SMN2, detect the familial deletion or sequence variant, control for maternal-cell contamination, and determine SMN2 copy number if the fetus is affected. SMN2 count can inform broad expectations but cannot predict exact severity or treatment response.
Cell-free DNA screening is not a substitute for diagnostic testing for SMA. Commercial offerings and validation differ, and a screening estimate does not provide the same certainty as chorionic villus sampling or amniocentesis.
If only one partner is a known carrier and the other has a reduced-risk two-copy result, invasive prenatal diagnosis is not automatically required. The couple should receive their calculated residual risk and discuss how they feel about the remaining uncertainty, procedure risks, and whether fetal results would change pregnancy or delivery planning.
A fetus found to have no functional SMN1 copies should be referred for multidisciplinary counseling. Families need current information about newborn screening, presymptomatic treatment, available therapies, and the limits of genotype-based prognosis. Historical natural-history descriptions may not reflect outcomes with early modern treatment.
Delivery planning may include rapid confirmatory testing and connection to a pediatric neuromuscular center. Treatment decisions after birth are time-sensitive, so prenatal knowledge can shorten delays.
General information about prenatal genetic testing can help couples prepare questions, but the laboratory plan must be tailored to the family’s SMN1 findings.
Carrier testing versus diagnostic SMA testing
Carrier screening and diagnostic testing use related methods but answer different questions.
Carrier screening asks whether an unaffected person can pass a nonworking SMN1 allele. It usually reports SMN1 copy number and may report silent-carrier markers. It may not sequence every part of SMN1.
Diagnostic testing is used for a person with symptoms, a positive newborn screen, or a prenatal result suggesting SMA. Most newborn-screening programs detect the common homozygous absence of SMN1 exon 7. They may not detect compound heterozygous SMA in which one chromosome has a deletion and the other has a rare sequence variant.
A diagnostic workup may include:
- SMN1 exon 7 copy-number testing
- SMN1 sequencing when one copy remains and symptoms are compatible
- SMN2 copy number for prognosis and treatment planning
- Neurologic examination and motor assessment
- Electrophysiology or broader neuromuscular testing when results are discordant
A person with zero SMN1 copies and compatible findings has a molecular diagnosis of 5q SMA. A person with one SMN1 copy and a pathogenic sequence variant in the remaining copy can also have SMA. A person with two SMN1 copies is unlikely to have typical 5q SMA but may need sequencing, phase analysis, or evaluation for another neuromuscular disorder if symptoms are strong.
SMN2 copy number should not be used to decide that an infant is “too mild” for urgent referral. Presymptomatic treatment can produce much better outcomes than treatment after motor neuron loss. Newborn-screen-positive infants need rapid confirmatory testing and specialist action.
Conversely, an adult carrier with one SMN1 copy does not need SMA treatment or SMN2-based prognosis. Carrier status is a reproductive finding, not a diagnosis of motor neuron disease.
A diagnostic genetic result guide can help distinguish disease testing from screening, but the SMN region’s copy-number complexity requires specific counseling.
Limitations and next steps
Before accepting a result, check whether the report answers these questions:
- How many SMN1 exon 7 copies were detected?
- Was exon 8 also assessed, and were discordant configurations considered?
- Was c.*3+80T>G or another duplication marker tested?
- Is the marker result used to provide residual risk?
- Does the assay detect SMN1 sequence variants or only copy number?
- Was SMN2 copy number reported, and is it being used appropriately?
- Does family history require testing beyond the standard screen?
Copy-number assays can be affected by rare sequence changes under primer or probe sites, gene conversion between SMN1 and SMN2, complex hybrid genes, and unusual copy arrangements. Laboratories use multiple probes or confirmatory methods to reduce these risks, but no assay is perfect.
A prior allogeneic bone marrow or stem-cell transplant can make blood DNA reflect the donor. For germline carrier testing, the laboratory may request cultured skin fibroblasts or another non-hematopoietic sample. Saliva can contain blood-derived cells and should be discussed with the laboratory rather than assumed to solve the problem.
If a person has one SMN1 copy and a family history of SMA, testing may already be sufficient to identify carrier status, but the familial sequence variant should still be documented. If a person has two copies but a sibling is a known 2+0 carrier, parental or family phase studies may be more useful than repeating a generic screen.
Reports can change as marker data and risk estimates improve. Keep the original document with copy number, marker status, laboratory method, and date. A simple note saying “SMA negative” loses the information needed for future pregnancy care.
Do not compare residual-risk numbers from different laboratories without checking their ancestry categories and assumptions. A number such as 1 in 500 is an estimate, not a measured property of one person. The clinically useful comparison is the couple’s combined risk and whether additional testing could materially change it.
If both partners have concerning results, referral to genetic counseling should not wait until late pregnancy. If a newborn screen is positive or an infant has weakness, poor feeding, reduced movement, absent reflexes, or breathing difficulty, urgent neuromuscular evaluation is needed. Carrier-screening discussions should never delay diagnostic care for symptoms.
References
- Spinal Muscular Atrophy. 2024. GeneReviews.
- Recommendations for Interpreting and Reporting Silent Carrier and Disease-Modifying Variants in SMA Testing Workflows. 2022. Review and reporting recommendations.
- Screening for autosomal recessive and X-linked conditions during pregnancy and preconception: a practice resource of the American College of Medical Genetics and Genomics (ACMG). 2021. Practice resource.
- A Five-Year Review of Newborn Screening for Spinal Muscular Atrophy in the United States. 2024. Original research.
- Comprehensive copy number analysis of spinal muscular atrophy genes using multiplex ligation-dependent probe amplification. 2024. Original research.
Disclaimer
This article provides general education and does not calculate an individual or couple’s SMA risk or replace laboratory-specific counseling. SMN1 copy number, duplication markers, family history, sequence variants, and residual-risk assumptions should be reviewed by qualified genetics professionals. A positive newborn screen or an infant with weakness, reduced movement, feeding difficulty, or breathing problems requires urgent clinical evaluation.
