SMN1 rs121908747: Understanding Silent Carrier Status
The variant rs121908747, often referred to as c.*3+80T>G, is a genetic marker located within the SMN1 gene. It is primarily used in clinical genetics to help identify 'silent carriers' of spinal muscular atrophy (SMA) who possess two copies of the SMN1 gene on a single chromosome.
What each genotype means
Typical residual carrier risk
You do not carry the linked marker associated with duplicated SMN1 alleles on a single chromosome. If clinical testing determines you have two SMN1 copies, this genotype does not elevate your baseline statistical risk of being a 2+0 silent carrier. However, it does not completely eliminate the residual risk of carrying a rare deletion or intragenic mutation.
Found in the vast majority of individuals across all major continental ancestral backgrounds.
Increased silent carrier probability
You carry one copy of the linked marker associated with duplicated SMN1 genes arranged in cis. If diagnostic copy-number screening identifies two copies of SMN1, carrying this marker statistically elevates the likelihood that both copies reside on one chromosome while the other chromosome carries an SMN1 deletion. The exact residual risk modification varies markedly by ancestral background and is highest in Ashkenazi Jewish and Asian populations.
Present in roughly 1% to 4% of individuals depending on ancestry, with higher frequencies observed in select cohorts.
Elevated silent carrier probability
You carry two copies of the linked marker associated with tandem SMN1 gene duplication events. In individuals with two total SMN1 gene copies, this genotype substantially increases the mathematical probability of a 2+0 silent carrier configuration. Comprehensive carrier risk assessment and genetic counseling are recommended to interpret this result alongside formal SMN1 dosage quantification.
Very rare across global populations, occurring in fewer than 1 in 1,000 individuals.
What is rs121908747?
The variant rs121908747 is a single nucleotide polymorphism (SNP) located in the SMN1 gene, specifically at the position often cited as c.*3+80T>G. In the context of genetic screening, this variant serves as a critical diagnostic tool. While standard carrier screening for spinal muscular atrophy (SMA) typically relies on measuring the total copy number of the SMN1 gene, this method can miss individuals known as 'silent carriers.' These individuals carry two copies of the SMN1 gene on one chromosome and zero copies on the other (a 2+0 genotype). Because they have a total of two copies, they may appear to be non-carriers in basic tests. The presence of the G allele at this specific site is a strong indicator that an individual may be a silent carrier, allowing clinicians to refine risk assessments beyond simple copy number counts.
The Role of the SMN1 Gene
The SMN1 gene provides instructions for producing the survival motor neuron (SMN) protein, which is essential for the health and survival of motor neurons in the spinal cord. Motor neurons are specialized nerve cells that control muscle movement. When the SMN1 gene is missing or mutated, the resulting deficiency in SMN protein leads to the degeneration of these neurons, causing the muscle weakness and atrophy characteristic of SMA. The SMN1 gene is unique because it has a nearly identical paralog, SMN2. While SMN2 also produces the SMN protein, it does so inefficiently due to a small difference in its genetic sequence that causes the exclusion of exon 7. Consequently, SMN2 cannot fully compensate for the loss of SMN1, which is why the status of SMN1 is the primary determinant of SMA risk.
Research and Clinical Significance
Research into rs121908747 has transformed how laboratories approach SMA carrier screening. Historically, detecting silent carriers was nearly impossible without extensive pedigree analysis. However, studies have shown that the G allele at this position is highly associated with the 2+0 haplotype. By incorporating this marker into testing workflows, laboratories can significantly increase the detection rate of silent carriers, which are estimated to account for 1% to 5% of all SMA carriers. While the evidence for using this marker is considered moderate to strong in clinical settings, it is not a standalone diagnostic. It is used in conjunction with quantitative copy number analysis to provide a more comprehensive picture. As with all genetic testing, these results should be interpreted by a qualified genetic counselor or medical professional who can explain the implications for family planning and personal health.
What You Can Do With This Information
If you receive information about your status regarding rs121908747, it is important to understand that this is a screening marker, not a diagnosis of a condition. This information is most commonly used in the context of reproductive carrier screening to assess the risk of having a child with SMA. If you are identified as a potential carrier, the most appropriate next step is to consult with a genetic counselor. They can help you understand your specific results, discuss the limitations of the testing, and explain what this means for your family. You should never make medical decisions based solely on a genetic report without professional guidance. If you have questions about how this variant relates to your health or family history, bring your report to your primary care physician or a specialist who can provide context based on your full clinical profile.
How common is this variant?
The frequency of the silent carrier haplotype varies significantly by ethnicity, and the specific marker rs121908747 is used to identify these individuals across diverse populations.
Frequently asked questions
What is a silent carrier of SMA?
A silent carrier has two copies of the SMN1 gene on one chromosome and zero copies on the other. Because they have two copies total, they may test as 'normal' on standard screening tests that only count gene copies.
Does having the G allele mean I have SMA?
No. This variant is a marker used to identify carrier status, not to diagnose the disease itself. Being a carrier does not mean you have or will develop spinal muscular atrophy.
How accurate is testing for this variant?
Testing for this marker significantly improves the detection of silent carriers compared to copy number analysis alone. However, no genetic test is 100% accurate, and results should always be discussed with a genetic counselor.
Should I be worried if I am a carrier?
Being a carrier is common and does not affect your own health. It is primarily relevant for family planning, as it helps determine the risk of passing the condition to future children.
Sources & further reading
Educational information only, last refreshed 10/28/2026. Not medical advice — these associations describe population statistics, not individual predictions.
Curious what your genotype is for rs121908747?
Upload a raw DNA file from 23andMe, AncestryDNA, MyHeritage, or FamilyTreeDNA and see this variant — plus thousands more — interpreted in your full report.
Get my report — $29