Skip to content
How It Works
Explore
Ask a Question Resources Pricing
Who It's For
Patients
Decode results, prep for appointments, weigh options.
Caregivers & advocates
Carry the research load for someone you love.
Providers & clinicians
Resources for patients in your practice.
Log In Sign Up
PubMed This is a summary of 15 peer-reviewed journal articles Updated
Neurology

Genetic Counseling and Family Implications

At a Glance

A C9ORF72 expansion is inherited in an autosomal dominant pattern, so each child of a carrier has a 50% chance of inheriting it. However, not everyone who inherits it becomes ill, and symptoms may range from a Huntington-like movement disorder to ALS or FTD, making genetic counseling important.

Because the C9ORF72 expansion is a genetic condition, your diagnosis is a “family affair.” Understanding how this gene is passed down and what it means for your relatives is one of the most complex parts of the journey.

This condition follows an autosomal dominant inheritance pattern [1]. This means that if a parent carries the expansion, each of their biological children has a 50% (1 in 2) chance of inheriting it [2]. However, unlike classic Huntington’s disease, inheriting the gene does not mean the person is “guaranteed” to get the exact same symptoms—or even get sick at all [3].

The Two Rules of C9ORF72 Genetics

To understand why this diagnosis looks different in every family, experts focus on two key concepts:

  1. Incomplete Penetrance: Not everyone who inherits the C9ORF72 expansion will develop symptoms [3]. Some people carry the expansion into their 70s, 80s, or beyond without any signs of neurological disease [4]. Penetrance estimates vary substantially depending on family history, age, and whether ALS, FTD, or overlap syndromes are being counted [3]. A counselor can discuss family-specific but still imprecise age-related risk.
  2. Variable Expressivity: Even within the same family, the expansion can act differently [5]. You may have a Huntington’s-like movement disorder, but your child or sibling who inherits the same gene might develop ALS (muscle weakness) or FTD (personality changes) instead [6][7].

Diagnostic vs. Predictive Testing

There is a big difference between the testing you had and the testing your healthy relatives might consider:

  • Diagnostic Testing: This is what you underwent. It is used to find the cause of symptoms that are already present [8].
  • Predictive Testing: This is generally offered to competent adults who want to know if they carry the expansion before any symptoms appear [9]. Testing asymptomatic minors for an adult-onset condition is usually deferred unless there is a specific medical reason.

The Importance of Genetic Counseling

Because a positive result cannot predict when a person will get sick, which disease they will get, or how severe it will be, predictive testing is a major life decision [10]. Relatives considering testing should work with a genetic counselor [8].

A counselor helps families navigate:

  • The “Right to Know” vs. “Right Not to Know”: Some people find that knowing their status helps them plan their lives or make reproductive choices, while others find the uncertainty of a positive result too heavy to bear [11][12].
  • Psychological Readiness: While there is no single universally adopted predictive-testing protocol for this expansion, centers strongly advise pretest counseling, informed consent, psychosocial assessment, and post-test support [8]. This ensures the person has the coping skills needed to handle the result, as responses can range from relief to significant anxiety or grief [13][14].
  • Privacy and Discrimination: Counselors discuss how a genetic result might affect insurance. In the United States, GINA generally covers health insurance and employment, but not life, disability, or long-term-care insurance, with important exceptions and state-law variation [15].
  • Reproductive Options: For those planning a family, a counselor can explain options like preimplantation genetic testing (PGT), which can select embryos without the familial expansion. However, it does not guarantee a pregnancy or healthy child and involves significant availability, cost, legal, and ethical considerations [11][13].

Ultimately, the goal of genetic counseling is not to tell your relatives whether to get tested, but to empower them to make the choice that is right for their own mental health and future [13].

Common questions in this guide

What is the chance that my child will inherit a C9ORF72 expansion?
C9ORF72 expansions follow an autosomal dominant inheritance pattern. If one parent carries the expansion, each biological child has a 50% chance of inheriting it. Inheriting it does not guarantee that the child will develop symptoms.
Can someone carry a C9ORF72 expansion without becoming ill?
Yes. Incomplete penetrance means that some people who inherit the expansion never develop neurological symptoms, even into older age. The chance of developing disease varies with age and family history, and current testing cannot provide a precise personal prediction.
What diseases can a C9ORF72 expansion cause?
The same expansion can be associated with a Huntington-like movement disorder, amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), or an overlap of these conditions. This difference between relatives is called variable expressivity, meaning the same genetic change can produce different symptoms.
How is predictive genetic testing different from diagnostic testing?
Diagnostic testing looks for the cause of symptoms that are already present. Predictive testing is considered by adults who do not have symptoms but want to learn whether they carry the expansion. Testing children for an adult-onset condition is usually deferred unless there is a specific medical reason.
Why should I meet with a genetic counselor before predictive testing?
A positive result cannot reliably predict when symptoms will begin, which condition will develop, or how severe it will be. A genetic counselor can review informed consent, emotional readiness, privacy and insurance issues, family communication, and reproductive choices before and after testing.
Does GINA protect all insurance coverage after a C9ORF72 result?
In the United States, the Genetic Information Nondiscrimination Act generally addresses health insurance and employment discrimination, but it does not generally cover life, disability, or long-term-care insurance. Exceptions and state laws vary, so people should discuss current rules with a qualified counselor or legal professional before testing.
Can families use reproductive testing to avoid passing on the expansion?
Preimplantation genetic testing can help select embryos without the familial C9ORF72 expansion. It does not guarantee pregnancy or a healthy child, and availability, cost, legal, and ethical considerations vary.

Get notified when new evidence is published on Huntington disease-like syndrome due to C9ORF72 expansions.

We monitor PubMed for new peer-reviewed studies on this topic and email a short summary when something meaningful changes.

References

References (15)
  1. 1

    Frequency of C9orf72, GRN, and MAPT pathogenic variants in patients recruited at the Belgrade Memory Center.

    Stefanova E, Marjanović A, Dobričić V, et al.

    Neurogenetics 2024; (25(3)):193-200 doi:10.1007/s10048-024-00766-8.

    PMID: 38847891
  2. 2

    Age-based risk estimates for C9orf72RE-related diseases: Theoretical developments and added value for genetic counseling.

    de Vienne D, de Vienne DM

    PLoS genetics 2026; (22(7)):e1012230 doi:10.1371/journal.pgen.1012230.

    PMID: 42441689
  3. 3

    Assessment of risk of ALS conferred by the GGGGCC hexanucleotide repeat expansion in C9orf72 among first-degree relatives of patients with ALS carrying the repeat expansion.

    Van Wijk IF, Van Eijk RPA, Van Boxmeer L, et al.

    Amyotrophic lateral sclerosis & frontotemporal degeneration 2024; (25(1-2)):188-196 doi:10.1080/21678421.2023.2272187.

    PMID: 37861203
  4. 4

    C9ORF72 Gene GGGGCC Hexanucleotide Expansion: A High Clinical Variability from Amyotrophic Lateral Sclerosis to Frontotemporal Dementia.

    Kortazar-Zubizarreta I, Manero-Azua A, Afonso-Agüera J, Perez de Nanclares G

    Journal of personalized medicine 2023; (13(9)) doi:10.3390/jpm13091396.

    PMID: 37763163
  5. 5

    C9orf72 expansions in frontotemporal dementia and amyotrophic lateral sclerosis.

    Rohrer JD, Isaacs AM, Mizielinska S, et al.

    The Lancet. Neurology 2015; (14(3)):291-301.

    PMID: 25638642
  6. 6

    Expanding the spectrum of C9ORF72-related neurodegenerative disorders in the Greek population.

    Kartanou C, Kontogeorgiou Z, Rentzos M, et al.

    Journal of the neurological sciences 2022; (442()):120450 doi:10.1016/j.jns.2022.120450.

    PMID: 36252286
  7. 7

    Unravelling the clinical spectrum and the role of repeat length in C9ORF72 repeat expansions.

    van der Ende EL, Jackson JL, White A, et al.

    Journal of neurology, neurosurgery, and psychiatry 2021; (92(5)):502-509 doi:10.1136/jnnp-2020-325377.

    PMID: 33452054
  8. 8

    C9orf72 and the Care of the Patient With ALS or FTD: Progress and Recommendations After 10 Years.

    Roggenbuck J

    Neurology. Genetics 2021; (7(1)):e542 doi:10.1212/NXG.0000000000000542.

    PMID: 33575483
  9. 9

    Current knowledge and recent insights into the genetic basis of amyotrophic lateral sclerosis.

    Volk AE, Weishaupt JH, Andersen PM, et al.

    Medizinische Genetik : Mitteilungsblatt des Berufsverbandes Medizinische Genetik e.V 2018; (30(2)):252-258 doi:10.1007/s11825-018-0185-3.

    PMID: 30220791
  10. 10

    The C9orf72 hexanucleotide repeat expansion presents a challenge for testing laboratories and genetic counseling.

    Crook A, McEwen A, Fifita JA, et al.

    Amyotrophic lateral sclerosis & frontotemporal degeneration 2019; (20(5-6)):310-316 doi:10.1080/21678421.2019.1588904.

    PMID: 30907153
  11. 11

    Genetic counseling and testing for dementia - A scoping review of patient and relatives experiences and outcomes.

    Chen G, Sexton A

    Patient education and counseling 2026; (143()):109424 doi:10.1016/j.pec.2025.109424.

    PMID: 41289740
  12. 12

    Predictive genetic testing for amyotrophic lateral sclerosis and frontotemporal dementia: genetic counselling considerations.

    Crook A, Williams K, Adams L, et al.

    Amyotrophic lateral sclerosis & frontotemporal degeneration 2017; (18(7-8)):475-485 doi:10.1080/21678421.2017.1332079.

    PMID: 28585888
  13. 13

    Patient and Relative Experiences and Decision-making About Genetic Testing and Counseling for Familial ALS and FTD: A Systematic Scoping Review.

    Crook A, Jacobs C, Newton-John T, et al.

    Alzheimer disease and associated disorders 2021; (35(4)):374-385 doi:10.1097/WAD.0000000000000458.

    PMID: 34054018
  14. 14

    Genetic testing for monogenic forms of motor neuron disease/amyotrophic lateral sclerosis in unaffected family members.

    Howard J, Chaouch A, Douglas AGL, et al.

    European journal of human genetics : EJHG 2025; (33(1)):7-13 doi:10.1038/s41431-024-01718-4.

    PMID: 39501102
  15. 15

    The ethical implications of genetic testing in neurodegenerative diseases: A systematic review.

    Nurmi SM, Halkoaho A, Moilanen J, et al.

    Scandinavian journal of caring sciences 2021; (35(4)):1057-1074 doi:10.1111/scs.12932.

    PMID: 33210792

This page is for informational purposes only and does not constitute medical advice. A genetic counselor or neurologist can help interpret C9ORF72-related family risk and testing choices for your situation.

Guide contents Huntington disease-like syndrome due to C9ORF72 expansions
Research & Literature
04 Treatment Strategy and Symptom Management 06 Long-Term Monitoring and Future Directions