Huntington disease

What causes Huntington disease?

Huntington disease (HD) is caused by a change (mutation) in the HTT gene. This gene gives instructions for making a protein called huntingtin. The exact function of this protein is unclear, but it appears to be important to nerve cells (neurons) in the brain.

The HTT gene mutation that causes HD involves a DNA segment known as a CAG trinucleotide repeat. This segment is made up of three DNA building blocks that repeat multiple times in a row. The CAG segment in a normal HTT gene repeats about 10 to 35 times. In people with HD, it may repeat from 36 to over 120 times. People with 36 to 39 CAG repeats (an intermediate size) may or may not develop HD, while people with 40 or more repeats almost always develop HD.

Last updated on 05-01-20

How is Huntington disease diagnosed?

A diagnosis of Huntington disease is typically suspected in people with characteristic signs and symptoms of the condition and a family history consistent with autosomal dominant inheritance. The diagnosis can then be confirmed with genetic testing that identifies a specific type of change (mutation) in the HTT gene.

Last updated on 05-01-20

How is Huntington disease inherited?

Huntington disease (HD) is inherited in an autosomal dominant manner. This means that having a change (mutation) in only one of the 2 copies of the HTT gene is enough to cause the condition. When a person with HD has children, each child has a 50% (1 in 2) chance to inherit the mutated gene and develop the condition.

Most people with HD have an affected parent. The family history can sometimes appear negative for various reasons even though a parent carries, or carried, a mutation in the HTT gene. In rare cases, HD is caused by a new (de novo) mutation in the HTT gene, in which case the disease occurs for the first time in the affected person and is not inherited from a parent.

As HD is passed through generations, the size of the mutation in the HTT gene (called a trinucleotide repeat) often increases. A longer repeat in the HTT gene may cause earlier onset of symptoms. This phenomenon is called anticipation.

Last updated on 05-01-20

What tests will my doctor order to confirm that I have Huntington disease?

Genetic testing for a change (mutation) in the HTT gene can be ordered to confirm a diagnosis of Huntington disease.

It is important to bear in mind that, even though a person may be aware that he or she has some of the symptoms associated with Huntington disease, hearing this suspicion confirmed can often have a significant psychological effect. For this reason, you are strongly encouraged to bring a support person with you to all testing sessions. You may also wish to have your testing done at an Huntington Disease Society of America (HDSA) Center of Excellence where there are resources available to guide you through the testing process. To locate a Center of Excellence in your area, please click here.

Last updated on 05-01-20

Who can I contact to learn more about genetic disease and environmental exposure?

We recommend that you contact the National Institute of Environmental Health Sciences (NIEHS). The NIEHS strives to reduce the burden of human illness and dysfunction from environmental causes by defining how environmental exposures, genetic susceptibility, and age interact to affect an individual's health.

National Institute of Environmental Health Sciences
P.O. Box 12233
Research Triangle Park, NC 27709
Telephone: 919-541-3345
TTY: 919-541-4644
Web site:

Last updated on 05-01-20

What is Huntington disease?

Huntington disease (HD) is an inherited condition that causes progressive degeneration of neurons in the brain. Signs and symptoms usually develop between ages 35 to 44 years and may include uncontrolled movements, loss of intellectual abilities, and various emotional and psychiatric problems. People with HD usually live for about 15 to 20 years after the condition begins. It is caused by changes (mutations) in the HTT gene and is inherited in an autosomal dominant manner. Treatment is based on the symptoms present in each person and may include various medications.

There is also a less common, early-onset form of HD which begins in childhood or adolescence. For more information on this form, please visit GARD's juvenile Huntington disease Web page.

Last updated on 05-01-20

Can a person have 17 CAG repeats on both copies of their HD gene?

Yes. While the number of CAG copies is highly variable, it is quite possible for a person to inherit two 17 CAG repeats. It is however also possible for a HD genetic test to fail to detect one of a persons two HD genes. This may be because the gene itself has changes in it that prevent the test from working. These changes have no bearing on the CAG repeat length itself, but can prevent the test from detecting the gene. Another reason for failed detection of one of the genes, is that one of the copies is extremely large. HD genes with extremely large CAG repeat sizes may also be missed by initial testing, but can be detected with additional testing..

The laboratory that preformed your loved one's test is an essential source for information regarding the reason for her inconclusive test result. If she has not already done so, we strongly recommend that she discuss her concerns further with her healthcare provider.

Last updated on 05-01-20

Can a person of any gender, age, or ethnicity have Huntington disease?

Huntington disease (HD) affects both men and women of all ethnic groups. However, the frequency of the condition in different countries varies greatly. In general, it affects about 3 to 7 per 100,000 people of western European descent. A few isolated populations of western European origin have an unusually high prevalence of HD that appears to have resulted from a founder effect. For example, the Lake Maracaibo region of Venezuela is believed to have the highest prevalence of HD in the world with about 700 per 100,000 affected. The condition occurs less frequently in Japan, China, and Finland, as well as among black people of African descent.

The average age of onset for HD is 35 to 44 years. However, the range is large and varies from 2 years to older than 80 years. Onset in people younger than 10 years and in people older than 70 years is rare.

Last updated on 05-01-20

What are the exercise recommendations for people with Huntington disease?

The National Institute of Neurological Disorders and Stroke (NINDS) states that, "It is extremely important for the person with HD to maintain physical fitness as much as his or her condition and the course of the disease allows. Individuals who exercise and keep active tend to do better than those who do not." In 2010, the Huntington's Disease Society of America published a booklet on Physical and Occupational Therapy that provides information on therapy for the different stages of HD. The Huntington Society of Canada also provides information in their Healthcare Professionals Series: Exercise for People with Huntington Disease.

Despite these recommendations, there has been conflicting research about exercise and HD. Some research has shown exercise can be beneficial, while other research has shown exercise could be harmful. The Huntington's Disease Society of America posted the article, "Exercise Found Harmful in a Mouse Model," which explains the conflicting research. It is very important for a person with HD to consult their doctor to develop an appropriate fitness plan.

Last updated on 05-01-20

Can people without a family history of Huntington disease still develop the condition?

Yes. While most people with Huntington disease (HD) inherit it from an affected parent, in rare cases a person has HD due to a de novo mutation in the HTT gene (a mutation that occurs for the first time and is not inherited from a parent). However, the family history can sometimes appear negative for various reasons even though a parent carries, or carried, a mutation.

Last updated on 05-01-20

Why might an at-risk asymptomatic adult seek testing for Huntington disease?

If a diagnosis of Huntington disease (HD) is confirmed in a family, asymptomatic adult family members who are at risk of developing the condition may seek testing in order to make personal decisions regarding reproduction, financial matters, and career planning. Others may have different motivations including simply the 'need to know.'

Presymptomatic testing for HD usually involves pretest interviews in which the motives for requesting the test, the person's knowledge of Huntington disease, the possible impact of positive and negative test results, and neurologic status are assessed. Those seeking testing should be counseled about possible problems that they may encounter with regard to health, life, and disability insurance coverage; employment and educational discrimination; and changes in social and family interaction. Informed consent should be obtained and records kept confidential. People who are found to carry a Huntington disease-causing gene mutation need arrangements for long- term follow-up and evaluations.

Last updated on 05-01-20

Should someone with a close family history of Huntington disease whose initial test results shows CAG repeats of the same size have additional testing?

Yes. While we know how very frustrating this is, and how very difficult it must be to wait for follow-up test results. In this scenario a person's doctor will recommend additional testing to confirm the test results.

If your loved one is not already doing so, we strongly recommend that she work with a genetics professional. Genetics professionals are a source of information regarding genetic testing for Huntington disease. More information about finding and visiting a genetics professional is available from the Genetic Consultation Handbook developed by Genetics Home Reference.

To find a genetics clinic, we recommend that you contact your primary healthcare provider for a referral. The following online resources can also help you find a genetics professional in your community:

You can also find HD genetic testing centers through the Huntington's Disease Society of America's Web site.

Last updated on 05-01-20

Are there certain chemicals or substances that people with Huntington disease should avoid?

Yes. There are certain compounds that can be particularly problematic for people with Huntington disease, such as L-dopa-containing compounds which may increase chorea; alcohol; and smoking. If you or someone you know has Huntington disease, we recommend that you speak with a healthcare provider to learn more about what substances to avoid.

Last updated on 05-01-20

Is genetic testing available for Huntington disease?

Yes. Testing of adults at risk for Huntington disease (HD) who have no symptoms of the disease is called predictive testing. Whether to have predictive testing requires careful thought, including pre-test and post-test genetic counseling. This is particularly important because there is currently no cure. Furthermore, predictive testing cannot accurately predict the age a person with an HD mutation will develop symptoms, the severity or type of symptoms they will experience, or the future rate of disease progression. A person may want to have predictive testing because they feel they need to know, or to make personal decisions involving having children, finances, and/or career planning. Other people decide they do not want to know whether they will develop HD.

Testing is appropriate to consider in symptomatic people of any age in a family with a confirmed diagnosis of HD. However, testing of asymptomatic people younger than age 18 is not considered appropriate. A main reason is that it takes away the choice of whether the person wants to know, while there is no major benefit to knowing at that age.

People who are interested in learning more about genetic testing for HD should speak with a genetics professional.

Last updated on 05-01-20

Can Huntington disease skip a generation?

Huntington disease (HD) does not occur in one generation, skip the next, and then reoccur in a subsequent generation. However, HD may appear to skip a generation for one of the following reasons:

  • Failure to recognize the disease in family members
  • Early death of a parent before the onset of symptoms
  • The presence of an intermediate allele (an allele with 27-35 CAG repeats) or an HTT allele with reduced penetrance (with 36-39 CAG repeats), which may or may not result in HD in an asymptomatic parent
  • Late onset of the disease in an affected parent

All people who inherit an allele with more than 40 CAG repeats will develop HD. Those with 36-39 repeats are considered at risk of developing the disease.

People that do not inherit the mutated gene from an affected parent are not at risk to develop HD, and because they don't carry the mutated gene, they are also not at risk to pass it on to their children.

Genetic counseling is strongly recommended for people with questions about the risk of HD for themselves or family members, or people who are considering genetic testing for a family history of HD.

Last updated on 05-01-20

Can environmental exposures cause gene expansion like that of the CAG repeat that causes Huntington disease?

We were unable to find information in the medical literature regarding an association between exposure to vinyl chloride (or other substances) and CAG repeat expansion or Huntington disease. However, we identified the following articles which may be of interest to you. The first one listed below discusses the differences in the age of onset of Huntington disease in identical twins and the possible role of environmental factors. The second article discusses how certain gene changes in combination with environmental exposures may have a role in causing disease.

Freidman JH et al., Monozygotic Twins Discordant for Huntington Disease After 7 Years. Arch Neurol. 2005;62:995-997.

Christiani DC, Mehta AJ, Yu CL. Genetic susceptibility to occupational exposures. Occup Environ Med. 2008 Jun;65(6):430-6; quiz 436, 397.

Last updated on 05-01-20

What is the long-term outlook for people with Huntington disease?

Huntington disease (HD) is progressive, eventually leading to disability and death (usually from a coexisting illness or infection). However, the disease affects everyone differently; the age of onset, specific symptoms, and rate of progression varies for each person with HD.

While the symptoms of HD are well-characterized, their progression (especially in the early and middle stages) remains unpredictable. With the approach of late-stage HD, affected people have speech difficulty and weight loss. In the late stage, affected people lose bowel and bladder control.

The duration of the disease (from onset until death) varies considerably, with an average of approximately 19 years. Most people with HD survive for 10-25 years after the onset of symptoms. The average age at death ranges from 51-57 years, but the range may be broader. In a large study, pneumonia and cardiovascular (heart) disease were the most common primary causes of death.

The length of the CAG repeat (the type of mutation in the HTT gene responsible for HD) is the most important factor that determines age of onset of HD. However, there is still a lot of variability. Both genetic and environmental factors are thought to play a role on the age of onset in people with a mutation. Inheritance through the father can lead to more repeat expansion and earlier onset through succeeding generations, a phenomenon called anticipation.

People with HD, family members, and/or caregivers with specific questions about prognosis should speak with their health care provider.

Last updated on 05-01-20

How might Huntington disease be treated?

Unfortunately, there is currently no cure for Huntington disease (HD). The current goal of treatment is to slow down the course of the disease and help affected people function for as long and as comfortably as possible.

Current treatment strategies involve the use of various medications to treat specific symptoms such as abnormal movements and behaviors. Depression and suicide are more common among affected people, so caregivers should monitor for associated symptoms and seek help if necessary. As symptoms of the disease worsen, affected people need more assistance, supervision, and care.

Last updated on 05-01-20

Testing Resources

Neurogenetics at the University of Washington

The Division of Neurogenetics at the University of Washington is a tertiary specialty that provides clinical services to adults in the general public as well as training. They have created a booklet entitled, "Huntington Disease: Making an Informed Choice, that can be downloaded at no charge.

Last updated on 04-27-20

Name: Huntington's Disease Society of America 505 Eighth Ave, Suite 902
New York, NY, 10018, United States
Phone: (212) 242-1968 Toll Free: (800) 345-HDSA (4372) Email: Url:
Name: International Huntington Association Callunahof 8 Harfsen, Intl 7217 ST
United Kingdom
Phone: 31-573-431595 Email: Url:
Name: Huntington Society of Canada 151 Frederick St, Suite 400 Kitchner, Ontario, Intl N2H 2M2
Phone: 519-749-7063 Toll Free: 800-998-7398 Fax : (519) 749-8965 Email: Url:
Name: Hereditary Disease Foundation 601 West 168th Street Suite 54
New York, NY, 10032, United States
Phone: 212-928-2172 Fax : 212-928-2172 Email: Url:
Name: HOPES Url:
Name: International Parkinson and Movement Disorder Society 555 East Wells Street, Suite 1100
Milwaukee, WI, 53202-3823, United States
Phone: +1-414-276-2145 Fax : +1-414-276-3349 Email: Url:
Name: CHDI Foundation 350 Seventh Ave, Suite 200
New York, NY, 10001 , United States
Phone: 212-239-9300 Fax : 212-239-2101 Email: Url:
Name: Parkinson & Movement Disorder Alliance PMD Alliance PO Box 36233
Tucson, AZ, 85704,
Toll Free: 1-800-256-0966 Email: Url:
Name: Huntington's Disease Youth Organization HDYO PO Box #272185
Boca Raton, FL, 33486,
Email: Url:

Note, these links are external searches against the National Laboratory of Medicine's drug database. You may need to adjust the search if there are no results found.

Drug Name Generic Name
Austedo d6-tetrabenazine, deutetrabenazine

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