C9orf72 rs139185008: Understanding This Genetic Marker
The rs139185008 variant is a single nucleotide polymorphism (SNP) located within the C9orf72 gene. It is primarily recognized as a genetic marker that tags the presence of hexanucleotide repeat expansions, which are associated with an increased risk for certain neurodegenerative conditions.
What each genotype means
Typical genetic profile
This genotype represents the common, non-risk version of this genetic marker. It is not associated with the hexanucleotide repeat expansion in the C9orf72 gene that is linked to certain neurodegenerative conditions.
This is the most common genotype found in the general population across most ancestries.
Potential expansion marker
This genotype is a known tagging marker for the C9orf72 hexanucleotide repeat expansion, which is associated with increased risk for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Research indicates that the frequency of this specific genotype decreases with age in some populations, suggesting a potential association with survival, though the evidence remains limited and is most robust in Finnish cohorts.
This genotype is rare in the general population, though it is more frequently observed in specific cohorts of Finnish ancestry.
Potential expansion marker
This genotype is a known tagging marker for the C9orf72 hexanucleotide repeat expansion, which is associated with increased risk for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Because this variant can act as a proxy for the repeat expansion, individuals with this genotype may have a higher statistical likelihood of carrying the expansion compared to those with the T/T genotype. Please consult with a genetic counselor or clinician if you have concerns regarding neurodegenerative disease risk.
This genotype is very rare in the general population, with higher prevalence observed primarily in individuals of Finnish descent.
What is rs139185008?
The rs139185008 variant is a specific change in the DNA sequence of the C9orf72 gene. In genetics, a SNP like this acts as a 'tag' or a signpost. Because it is frequently inherited alongside the C9orf72 hexanucleotide repeat expansion, researchers use this variant to identify individuals who may carry the expansion without needing to perform more complex and labor-intensive direct repeat-length testing. It sits within a region of the genome that has been extensively studied due to its link to motor neuron and cognitive health. While the SNP itself is a simple substitution of one DNA base for another, its primary clinical utility is its strong statistical correlation with the presence of the larger, more complex repeat expansion mutation.
The Role of the C9orf72 Gene
The C9orf72 gene provides instructions for producing a protein that is highly expressed in the brain, particularly in neurons and immune cells called microglia. This protein plays a critical role in maintaining the health of cells by regulating endolysosomal pathways, which are responsible for breaking down and recycling cellular waste. When the gene contains a large hexanucleotide repeat expansion—a sequence of six DNA building blocks repeated hundreds or thousands of times—it can lead to a loss of normal protein function, known as haploinsufficiency. This disruption can impair the ability of cells to respond to stress or damage, potentially contributing to the development of neurodegenerative conditions. Understanding this gene is a major focus of current research into how cellular maintenance failures lead to disease.
Research and Evidence Strength
The evidence linking rs139185008 to the C9orf72 hexanucleotide repeat expansion is strong, particularly in populations of European descent. Research has shown that this SNP is one of the most reliable markers for identifying carriers of the expansion in large biobank datasets. Beyond its role as a marker, some studies suggest that the C-allele of this variant may be associated with a higher risk of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), even in some cases where the massive expansion is not present, though this remains an area of active investigation. Because the evidence for its independent effect outside of the expansion is still evolving, the clinical significance of the variant is considered limited. It is important to note that genetic associations are statistical in nature and do not guarantee the development of any specific health condition.
Population Frequency
The rs139185008 variant is considered rare in the general population. Its frequency varies significantly by ancestry, with the most robust data coming from studies of Finnish and other European cohorts. Because it is a tag for a specific genetic expansion that is itself relatively uncommon, the variant is not typically found in high numbers across diverse global populations. Researchers often use large-scale databases like FinnGen or the UK Biobank to study its distribution. If you are interested in your own genetic data, it is important to remember that population-level statistics describe groups, not individuals. Rare variants often require specialized testing to confirm their presence and clinical relevance, and findings from one ancestral group may not apply directly to others.
What You Can Do With This Information
Information about genetic variants like rs139185008 is primarily intended for educational and research purposes. If you have received a report mentioning this variant, it is important to understand that it is a marker for a complex genetic feature, not a diagnostic test for a disease. You cannot use this information to diagnose yourself or predict your future health with certainty. If you have concerns about your family history of neurodegenerative conditions or your own health, the most appropriate step is to consult with a qualified healthcare provider or a genetic counselor. They can help you interpret your results in the context of your personal and family medical history and discuss whether further clinical testing is necessary. Never make medical decisions based solely on raw genetic data.
How common is this variant?
The rs139185008 variant is rare in the general population, with its highest documented frequencies observed in European cohorts, particularly within the Finnish population.
Frequently asked questions
Is rs139185008 a diagnostic test for ALS?
No, rs139185008 is not a diagnostic test. It is a genetic marker that tags the presence of a repeat expansion in the C9orf72 gene, which requires specialized clinical testing to confirm.
What does it mean if I have the C allele?
The C allele is statistically associated with the C9orf72 hexanucleotide repeat expansion. However, having the allele does not mean you have the expansion or will develop a disease; it simply indicates a higher probability of carrying the associated genetic feature.
Can I use this SNP to predict my risk of dementia?
Genetic markers like this provide statistical associations, not individual predictions. Neurodegenerative conditions are complex and influenced by many genetic and environmental factors, so this SNP alone cannot predict your personal risk.
Should I be worried if this variant appears in my DNA report?
Seeing this variant in a report should not be a cause for alarm. It is a common finding in genetic research, and you should discuss any concerns with a healthcare professional or genetic counselor who can provide context based on your health history.
Sources & further reading
Educational information only, last refreshed 11/2/2026. Not medical advice — these associations describe population statistics, not individual predictions.
Curious what your genotype is for rs139185008?
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 — $29Related variants in C9orf72
A tagging SNP for intermediate-length alleles of the C9orf72 repeat expansion, which has been investigated for its impact on healthy aging and longevity.
This SNP acts as an intermediate allele tag for the C9orf72 repeat expansion, which is linked to both ALS risk and longevity outcomes.