ALS Genetics & Gene Carriers
People who do not have ALS symptoms but come from a family affected by ALS or FTD may choose to learn more about predictive genetic testing and presymptomatic research.
This is different from testing someone who already has ALS.
Predictive testing can have medical, psychological, family, insurance, and reproductive implications.
A presymptomatic gene carrier is a person who carries a disease-associated genetic variant but does not currently have symptoms of ALS or another associated condition.
Presymptomatic carriers are especially important to research because they allow scientists to study disease biology before clinical symptoms appear.
Longitudinal studies can help researchers examine:
Sources: Pre-fALS Study
Inheritance risk and penetrance are different concepts.
Inheritance describes whether a genetic variant is passed from parent to child.
Penetrance describes the proportion of people carrying a disease-associated variant who eventually develop the associated condition.
Penetrance can:
A single penetrance percentage should not be used across all ALS genes.
For C9orf72 specifically, disease expression is highly variable. Carriers may develop ALS, FTD, both, or may remain unaffected for many years.
The presence of a C9orf72 repeat expansion does not allow clinicians to predict the exact disease course for an individual carrier.
Sources: C9orf72 Frontotemporal Dementia and/or Amyotrophic Lateral Sclerosis
Predictive testing is genetic testing performed in a person without symptoms to determine whether they inherited a familial disease-associated variant.
Testing is generally most informative when the disease-associated variant has first been identified in an affected family member.
Predictive testing should involve genetic counseling so the person understands:
Sources: Benefits and Risks of Genetic Testing for Family Members of People Who Have or Had ALS · Genetic Counseling
Learning genetic status can affect people differently.
Possible responses may include:
Genetic counseling can help people consider these possibilities before testing.
Sources: Benefits and Risks of Genetic Testing for Family Members of People Who Have or Had ALS
A result may provide information about biological relatives.
Testing decisions may therefore affect:
Different members of the same family may make different decisions about whether they want to know their genetic status.
Sources: Benefits and Risks of Genetic Testing for Family Members of People Who Have or Had ALS · Genetic Counseling
United States federal law provides some protections against genetic discrimination in health insurance and employment.
However, federal GINA protections do not generally extend to:
Insurance laws vary by state and jurisdiction.
Genetic testing should not be described as financially risk-free.
Sources: Genetic Discrimination
People who know they carry an inherited pathogenic variant may choose to discuss reproductive options with a genetics or reproductive specialist.
No particular reproductive choice is recommended here.
Sources: Genetic Counseling
Research opportunities for presymptomatic carriers may include:
Eligibility varies by gene, variant, age, symptoms, study location, biomarker status, and other protocol requirements.
The FDA-required confirmatory ATLAS trial evaluates tofersen in clinically presymptomatic adults with confirmed SOD1 mutations.
Sources & Further Reading
These source cards link directly to the original material. External websites open in a new tab.
ClinicalTrials.gov
Registry record for longitudinal research in people at genetic risk for ALS.
Visit sourceNational Library of Medicine / GeneReviews
A direct source for reviewing the medical, genetic, research, or policy information discussed on this page.
Visit sourceThe ALS Association
Discussion of predictive testing considerations for family members.
Visit sourceThe ALS Association
Practical overview of counseling before, during, and after ALS genetic testing.
Visit sourceNational Human Genome Research Institute
Federal overview of genetic-discrimination protections and their limits.
Visit sourceU.S. Food and Drug Administration
FDA follow-up information for accelerated-approval indications, including SOD1-associated ALS.
Visit sourceThis information is for education and does not replace individualized medical advice, genetic counseling, diagnosis, or treatment. Genetic risk, penetrance, and testing implications can differ by gene, variant, family, and individual.
Last reviewed: 09-30-2026
Medical information should be reviewed periodically as genetic research and clinical trials change.