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In 1872, the American physician George Huntington wrote about an illness that he called "an heirloom from generations away back in the dim past." He was not the first to describe the disorder, which has been traced back to the Middle Ages at least. One of its earliest names was chorea,* which, as in "choreography," is the Greek word for dance. The term chorea describes how people affected with the disorder writhe, twist, and turn in a constant, uncontrollable dance-like motion. Later, other descriptive names evolved. "Hereditary chorea" emphasizes how the disease is passed from parent to child. "Chronic progressive chorea" stresses how symptoms of the disease worsen over time. Today, physicians commonly use the simple term Huntington's disease (HD) to describe this highly complex disorder that causes untold suffering for thousands of families.

In the United States alone, about 30,000 people have HD; estimates of its prevalence are about 1 in every 10,000 persons. At least 150,000 others have a 50% risk of developing the disease and thousands more of their relatives live with the possibility that they, too, might develop HD.

Until recently, scientists understood very little about HD and could only watch as the disease continued to pass from generation to generation. Families saw the disease destroy their loved ones' ability to feel, think, and move. In the last several years, scientists working with support from the National Institute of Neurological Disorders and Stroke (NINDS) have made a significant number of breakthroughs in the area of HD research. With these advances, our understanding of the disease continues to improve.
This brochure presents information about HD, and about current research progress, to health professionals, scientists, caregivers, and, most importantly, to those already too familiar with the disorder: the many families who are affected by HD.

What Causes Huntington's Disease?
HD results from genetically programmed degeneration of brain cells, called neurons, in certain areas of the brain. This degeneration causes uncontrolled movements, loss of intellectual faculties, and emotional disturbance. Specifically affected are cells of the basal ganglia, structures deep within the brain that have a number of important functions, including coordinating movement. Within the basal ganglia, HD especially targets neurons of the striatum, particularly those in the caudate nuclei and the pallidum. Also affected is the brain's outer surface, or cortex, which controls thought, perception, and memory.

How is HD Inherited?
HD is found in every country of the world. It is a familial disease, passed from parent to child through a mutation or misspelling in the normal gene.

A single abnormal gene, the basic biological unit of heredity, produces HD. Genes are composed of deoxyribonucleic acid (DNA), a molecule shaped like a spiral ladder. Each rung of this ladder is composed of two paired chemicals called bases. There are four types of bases—adenine, thymine, cytosine, and guanine—each abbreviated by the first letter of its name: A, T, C, and G. Certain bases always "pair" together, and different combinations of base pairs join to form coded messages. A gene is a long string of this DNA that is composed of various combinations of A, T, C, and G. These unique combinations determine the gene's function, much like letters join together to form words. Each person has about 100,000 genes–three billion base pairs of DNA or bits of information repeated in the nuclei of human cells–which determine individual characteristics or traits.

Genes are arranged in precise locations along 23 rod-like pairs of chromosomes. One chromosome from each pair comes from an individual's mother, the other from the father. Each half of a chromosome pair is similar to the other, except for one pair, which determines the sex of the individual. This pair has two x chromosomes in females and one x and one y chromosome in males. The gene that produces HD lies on chromosome 4, one of the 22 non-sex-linked, or "autosomal," pairs of chromosomes, placing men and women at equal risk of acquiring the disease.

The impact of a gene depends partly on whether it is dominant or recessive. If a gene is dominant, then only one of the paired chromosomes is required to produce its called-for effect. If the gene is recessive, both parents must provide chromosomal copies for the trait to be present. HD is called an autosomal dominant disorder because only one copy of the defective gene, inherited from one parent, is necessary to produce the disease.

The genetic defect responsible for HD is a small sequence of DNA on chromosome 4 in which several base pairs are repeated many, many times. The normal gene has three DNA bases, composed of the sequence CAG. In people with HD, the sequence abnormally repeats itself dozens of times. Over time–and with each successive generation–the number of CAG repeats may expand further.

Each parent has two copies of every chromosome but gives only one copy to each child. Each child of an HD parent has a 50–50 chance of inheriting the HD gene. If a child does not inherit the HD gene, he or she will not develop the disease and cannot pass it to subsequent generations. A person who inherits the HD gene, and survives long enough, will sooner or later develop the disease. In some families, all the children may inherit the HD gene; in others, none do. Whether one child inherits the gene has no bearing on whether others will or will not share the same fate.

A small number of cases of HD are sporadic, that is, they occur even though there is no family history of the disorder. These cases are thought to be caused by a new genetic mutation-an alteration in the gene that occurs during sperm development and that brings the number of CAG repeats into the range that causes disease.

What Are the Major Effects of the Disease?
Early signs of the disease vary greatly from person to person. A common observation is that the earlier the symptoms appear, the faster the disease progresses.

Family members may first notice that the individual experiences mood swings or becomes uncharacteristically irritable, apathetic, passive, depressed, or angry. These symptoms may lessen as the disease progresses or, in some individuals, may continue and include hostile outbursts or deep bouts of depression.

HD may affect the individual's judgment, memory, and other cognitive functions. Early signs might include having trouble driving, learning new things, remembering a fact, answering a question, or making a decision. Some may even display changes in handwriting. As the disease progresses, concentration on intellectual tasks becomes increasingly difficult.

In some individuals, the disease may begin with uncontrolled movements in the fingers, feet, face, or trunk. These movements–which are signs of chorea–often intensify when the person is anxious. HD can also begin with mild clumsiness or problems with balance. Other persons develop choreic movements later on as the disease progresses. They may stumble or appear uncoordinated. Chorea often creates serious problems with walking, increasing the likelihood of falls.

The disease can progress to the point where speech is slurred and vital functions, such as swallowing, eating, speaking, and especially walking, continue to decline. Some individuals are unable to recognize others. Many, however, remain aware of their environment and are able to express emotions.

Some physicians have employed a recently developed Unified HD Rating Scale, or UHDRS, to assess the clinical features, stages, and course of HD. In general, the duration of the illness ranges from 10 to 30 years. The most common causes of death are infection (most often pneumonia), injuries related to a fall, or other complications.

At What Age Does HD Appear?
The rate of disease progression and the age of onset vary from person to person. Adult-onset or classic HD, with its disabling, uncontrolled movements, most often begins during middle age. There are, however, other variations of HD distinguished not just by age of onset but by a distinct array of symptoms. For example, some persons develop the disease as adults, but without chorea. They may appear rigid and move very little, or not at all, a condition called akinesia. These individuals are said to have akinetic-rigid HD or the Westphal variant of HD.

Some individuals develop symptoms of HD when they are very young—before age 20. The terms early-onset HD or juvenile HD are often used to describe HD that appears in a young person. A common sign of HD in a younger individual is a rapid decline in school performance. Symptoms can also include subtle changes in handwriting and slight problems with movement, such as slowness, rigidity, tremor, and rapid muscular twitching, called myoclonus. Several of these symptoms are similar to those seen in Parkinson's disease, and they differ from the chorea seen in individuals who develop the disease as adults. People with juvenile HD may also have seizures and mental disabilities. As mentioned previously, the earlier the onset of HD, the faster the disease seems to progress. The disease progresses most rapidly in individuals with juvenile or early-onset HD, and death often follows within 10 years.

It appears that individuals with juvenile HD have usually inherited the disease from their fathers. These individuals also tend to have the largest number of CAG repeats. Scientists believe that the reason for this may be found in the process of sperm production. Unlike eggs, sperm are produced in the millions. Because DNA is copied millions of times during this process, scientists theorize that there is an increased possibility for genetic mistakes to occur. To verify that there was a link between the number of CAG repeats in the HD gene and the age of onset of the disease, scientists studied a young boy who developed HD at the age of two, one of the youngest and most severe cases ever recorded. They found that he had the largest number of CAG repeats of anyone they had studied so far–nearly 100. The boy's case was central to the identification of the HD gene and at the same time helped confirm that juvenile patients with HD have the longest segments of CAG repeats, the only proven correlation between repeat length and age at onset.

A few individuals develop HD after age 55. Diagnosis in these persons can be very difficult. The symptoms of HD may be masked by other health problems, or the person may not display the severity of symptoms seen in individuals with an earlier onset of HD. These individuals may also show signs of depression rather than anger or irritability, or they may retain sharp control over their intellectual functions, such as memory, reasoning, and problem solving.

There is also a related complex called senile chorea. Some elderly individuals display the symptoms of HD, especially choreic movements, but have a normal gene and lack a family history of the disorder. Some scientists believe that a different gene mutation may account for this small number of cases. Others, however, believe senile chorea is a late-onset form of HD.

How Is HD Diagnosed?
The great American folk singer and composer Woody Guthrie died on October 3, 1967, after suffering from HD for 13 years. He had been misdiagnosed, considered an alcoholic, and shuttled in and out of mental institutions and hospitals for years before being properly diagnosed. His case, sadly, is not extraordinary, although the diagnosis can be made easily by experienced neurologists.

The discovery of the HD gene in 1993 resulted in a direct genetic test to make or confirm a diagnosis of HD in an individual who is exhibiting HD-like symptoms. Using a blood sample, the genetic test analyzes DNA for the HD mutation by counting the number of repeats in the HD gene region. Individuals who do not have HD usually have 28 or fewer CAG repeats. Individuals with HD usually have 40 or more repeats. A small%age of individuals, however, have a number of repeats that fall within a borderline region.

The physician will interview the individual intensively to obtain the medical history and rule out other conditions. He or she will perform a neurological examination including tests of the person's hearing, eye movements, strength, sensation, reflexes, balance, movement, and mental status, and will probably order a number of laboratory tests as well. Together, these tests form the neurological examination. In addition, the physician will ask about recent intellectual or emotional problems, which may be indications of HD.

In addition to direct testing, another tool used by physicians to diagnose HD is to take the family history, sometimes called a pedigree or genealogy. It is extremely important for family members to be candid and truthful with a doctor who is taking a family history.

People with HD commonly have impairments in the way the eye follows or fixes on a moving target. Abnormalities of eye movements vary from person to person and differ depending on the stage and duration of the illness.

The physician may ask the individual to undergo a brain imaging test. The computed tomography (CT) scanner provides an excellent image of brain structures with little if any discomfort. Those with HD may show shrinkage of some parts of the brain–particularly two areas known as the caudate nuclei and putamen–and enlargement of cavities within the brain called ventricles. These changes do not definitely indicate HD however, because they can also occur in other disorders. In addition, a person can have early symptoms of HD and still have a normal CT scan. When used in conjunction with a family history and record of clinical symptoms, however, CT can be an important diagnostic tool.

Other technologies for brain visualization, such as magnetic resonance imaging (MRI) and positron emission tomography (PET), are an important part of HD research efforts, but their usefulness to physicians trying to diagnose HD has not yet been established.

What is Presymptomatic Testing?
Presymptomatic testing is a method for identifying persons carrying the HD gene before symptoms appear. In the past, no laboratory test could positively identify people carrying the HD gene–or those fated to develop HD–before the onset of symptoms. That situation changed in 1983, when a team of scientists supported by the NINDS located the first genetic marker for HD–the initial step in developing a laboratory test for the disease.

A marker is a piece of DNA that lies near a gene and is usually inherited with it. Discovery of the first HD marker allowed scientists to locate the HD gene on chromosome 4. The marker discovery quickly led to the development of a presymptomatic test for some individuals, but this test required blood or tissue samples from both affected and unaffected family members in order to identify markers unique to that particular family. For this reason, adopted individuals, orphans, and people who had few living family members were unable to use the test.

Discovery of the HD gene has led to a less expensive, scientifically simpler, and far more accurate presymptomatic test that is applicable to the majority of at-risk people. The new test uses CAG repeat length to detect the presence of the HD mutation in blood. This is discussed further in the next section.

In a small number of individuals with HD–1 to 3%–no family history of HD can be found. Some individuals may not be aware of their genetic legacy, or a family member may conceal a genetic disorder from fear of social stigma. A parent may not want to worry children, scare them, or deter them from marrying. In other cases, a family member may die of another cause before he or she begins to show signs of HD. Sometimes, the cause of death for a relative may not be known, or the family is not aware of a relative's death. Adopted children may not know their genetic heritage, or early symptoms in an individual may be too slight to attract attention. These are among the many complicating factors that reflect the complexity of diagnosing HD.

How is the Presymptomatic Test Conducted?
An individual who wishes to be tested should contact the nearest testing center. (A list of such centers can be obtained from the Huntington Disease Society of America at 1–800-345-HDSA.) The testing process should include several components. Most testing programs include a neurological examination, pretest counseling, and follow-up. The purpose of the neurological examination is to determine whether or not the person requesting testing is showing any clinical symptoms of HD. It is important to remember that if an individual is showing even slight symptoms of HD, he or she risks being diagnosed with the disease during the neurological examination, even before the genetic test. During pretest counseling, the individual will learn about HD, about his or her own level of risk, and about the testing procedure. The person will be told about the test's limitations, the accuracy of the test, and possible outcomes. He or she can then weigh the risks and benefits of testing and may even decide at that time against pursuing further testing.

If a person decides to be tested, a team of highly trained specialists will be involved, which may include neurologists, genetic counselors, social workers, psychiatrists, and psychologists. This team of professionals helps the at-risk person decide if testing is the right thing to do and carefully prepares the person for a negative, positive, or inconclusive test result.

Individuals who decide to continue the testing process should be accompanied to counseling sessions by a spouse, a friend, or a relative who is not at risk. Other interested family members may participate in the counseling sessions if the individual being tested so desires.

The genetic testing itself involves donating a small sample of blood that is screened in the laboratory for the presence or absence of the HD mutation. Testing may require a sample of DNA from a closely related affected relative, preferably a parent, for the purpose of confirming the diagnosis of HD in the family. This is especially important if the family history for HD is unclear or unusual in some way.

Results of the test should be given only in person and only to the individual being tested. Test results are confidential. Regardless of test results, follow-up is recommended.

In order to protect the interests of minors, including confidentiality, testing is not recommended for those under the age of 18 unless there is a compelling medical reason (for example, the child is exhibiting symptoms).

Testing of a fetus (prenatal testing) presents special challenges and risks; in fact some centers do not perform genetic testing on fetuses. Because a positive test result using direct genetic testing means the at-risk parent is also a gene carrier, at-risk individuals who are considering a pregnancy are advised to seek genetic counseling prior to conception.

Some at-risk parents may wish to know the risk to their fetus but not their own. In this situation, parents may opt for prenatal testing using linked DNA markers rather than direct gene testing. In this case, testing does not look for the HD gene itself but instead indicates whether or not the fetus has inherited a chromosome 4 from the affected grandparent or from the unaffected grandparent on the side of the family with HD. If the test shows that the fetus has inherited a chromosome 4 from the affected grandparent, the parents then learn that the fetus's risk is the same as the parent (50–50), but they learn nothing new about the parent's risk. If the test shows that the fetus has inherited a chromosome 4 from the unaffected grandparent, the risk to the fetus is very low (less than 1%) in most cases.

Another option open to parents is in vitro fertilization with preimplantation screening. In this procedure, embryos are screened to determine which ones carry the HD mutation. Embryos determined not to have the HD gene mutation are then implanted in the woman's uterus.

In terms of emotional and practical consequences, not only for the individual taking the test but for his or her entire family, testing is enormously complex and has been surrounded by considerable controversy. For example, people with a positive test result may risk losing health and life insurance, suffer loss of employment, and other liabilities. People undergoing testing may wish to cover the cost themselves, since coverage by an insurer may lead to loss of health insurance in the event of a positive result, although this may change in the future.

With the participation of health professionals and people from families with HD, scientists have developed testing guidelines. All individuals seeking a genetic test should obtain a copy of these guidelines, either from their testing center or from the organizations listed on the card in the back of this brochure. These organizations have information on sites that perform testing using the established procedures and they strongly recommend that individuals avoid testing that does not adhere to these guidelines.

How Does a Person Decide Whether to be Tested?
The anxiety that comes from living with a 50% risk for HD can be overwhelming. How does a young person make important choices about long-term education, marriage, and children? How do older parents of adult children cope with their fears about children and grandchildren? How do people come to terms with the ambiguity and uncertainty of living at risk?
Some individuals choose to undergo the test out of a desire for greater certainty about their genetic status. They believe the test will enable them to make more informed decisions about the future. Others choose not to take the test. They are at peace with being at risk and with all that that may entail. There is no right or wrong decision, as each choice is highly individual. The guidelines for genetic testing for HD, discussed in the previous section, were developed to help people with this life-changing choice.

Whatever the results of genetic testing, the at-risk individual and family members can expect powerful and complex emotional responses. The health and happiness of spouses, brothers and sisters, children, parents, and grandparents are affected by a positive test result, as are an individual's friends, work associates, neighbors, and others. Because receiving test results may prove to be devastating, testing guidelines call for continued counseling even after the test is complete and the results are known.
Is There a Treatment for HD?
Physicians may prescribe a number of medications to help control emotional and movement problems associated with HD. It is important to remember however, that while medicines may help keep these clinical symptoms under control, there is no treatment to stop or reverse the course of the disease.
Antipsychotic drugs, such as haloperidol, or other drugs, such as clonazepam, may help to alleviate choreic movements and may also be used to help control hallucinations, delusions, and violent outbursts. Antipsychotic drugs, however, are not prescribed for another form of muscle contraction associated with HD, called dystonia, and may in fact worsen the condition, causing stiffness and rigidity. These medications may also have severe side effects, including sedation, and for that reason should be used in the lowest possible doses.

For depression, physicians may prescribe fluoxetine, sertraline hydrochloride, nortriptyline, or other compounds. Tranquilizers can help control anxiety and lithium may be prescribed to combat pathological excitement and severe mood swings. Medications may also be needed to treat the severe obsessive-compulsive rituals of some individuals with HD.
Most drugs used to treat the symptoms of HD have side effects such as fatigue, restlessness, or hyperexcitability. Sometimes it may be difficult to tell if a particular symptom, such as apathy or incontinence, is a sign of the disease or a reaction to medication.

What Kind of Care Does the Individual with HD Need?
Although a psychologist or psychiatrist, a genetic counselor, and other specialists may be needed at different stages of the illness, usually the first step in diagnosis and in finding treatment is to see a neurologist. While the family doctor may be able to diagnose HD, and may continue to monitor the individual's status, it is better to consult with a neurologist about management of the varied symptoms.

Problems may arise when individuals try to express complex thoughts in words they can no longer pronounce intelligibly. It can be helpful to repeat words back to the person with HD so that he or she knows that some thoughts are understood. Sometimes people mistakenly assume that if individuals do not talk, they also do not understand. Never isolate individuals by not talking, and try to keep their environment as normal as possible. Speech therapy may improve the individual's ability to communicate.

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. A daily regimen of exercise can help the person feel better physically and mentally. Although their coordination may be poor, individuals should continue walking, with assistance if necessary. Those who want to walk independently should be allowed to do so as long as possible, and careful attention should be given to keeping their environment free of hard, sharp objects. This will help ensure maximal independence while minimizing the risk of injury from a fall. Individuals can also wear special padding during walks to help protect against injury from falls. Some people have found that small weights around the ankles can help stability. Wearing sturdy shoes that fit well can help too, especially shoes without laces that can be slipped on or off easily.

Impaired coordination may make it difficult for people with HD to feed themselves and to swallow. As the disease progresses, persons with HD may even choke. In helping individuals to eat, caregivers should allow plenty of time for meals. Food can be cut into small pieces, softened, or pureed to ease swallowing and prevent choking. While some foods may require the addition of thickeners, other foods may need to be thinned. Dairy products, in particular, tend to increase the secretion of mucus, which in turn increases the risk of choking. Some individuals may benefit from swallowing therapy, which is especially helpful if started before serious problems arise. Suction cups for plates, special tableware designed for people with disabilities, and plastic cups with tops can help prevent spilling. The individual's physician can offer additional advice about diet and about how to handle swallowing difficulties or gastrointestinal problems that might arise, such as incontinence or constipation.

Caregivers should pay attention to proper nutrition so that the individual with HD takes in enough calories to maintain his or her body weight. Sometimes people with HD, who may burn as many as 5,000 calories a day without gaining weight, require five meals a day to take in the necessary number of calories. Physicians may recommend vitamins or other nutritional supplements. In a long-term care institution, staff will need to assist with meals in order to ensure that the individual's special caloric and nutritional requirements are met. Some individuals and their families choose to use a feeding tube; others choose not to.

Individuals with HD are at special risk for dehydration and therefore require large quantities of fluids, especially during hot weather. Bendable straws can make drinking easier for the person. In some cases, water may have to be thickened with commercial additives to give it the consistency of syrup or honey.

What Community Resources are Available?
Individuals and families affected by HD can take steps to ensure that they receive the best advice and care possible. Physicians and state and local health service agencies can provide information on community resources and family support groups that may exist. Possible types of help include:
Legal and Social Aid. HD affects a person's capacity to reason, make judgments, and handle responsibilities. Individuals may need help with legal affairs. Wills and other important documents should be drawn up early to avoid legal problems when the person with HD may no longer be able to represent his or her own interests. Family members should also seek out assistance if they face discrimination regarding insurance, employment, or other matters.
Home Care Services. Caring for a person with HD at home can be exhausting, but part-time assistance with household chores or physical care of the individual can ease this burden. Domestic help, meal programs, nursing assistance, occupational therapy, or other home services may be available from federal, state, or local health service agencies.
Recreation and Work Centers. Many people with HD are eager and able to participate in activities outside the home. Therapeutic work and recreation centers give individuals an opportunity to pursue hobbies and interests and to meet new people. Participation in these programs, including occupational, music, and recreational therapy, can reduce the person's dependence on family members and provides home caregivers with a temporary, much needed break.
Group Housing. A few communities have group housing facilities that are supervised by a resident attendant and that provide meals, housekeeping services, social activities, and local transportation services for residents. These living arrangements are particularly suited to the needs of individuals who are alone and who, although still independent and capable, risk injury when they undertake routine chores like cooking and cleaning.
Institutional Care. The individual's physical and emotional demands on the family may eventually become overwhelming. While many families may prefer to keep relatives with HD at home whenever possible, a long-term care facility may prove to be best. To hospitalize or place a family member in a care facility is a difficult decision; professional counseling can help families with this.

Finding the proper facility can itself prove difficult. Organizations such as the Huntington's Disease Society of America (see Information Resources) may be able to refer the family to facilities that have met standards set for the care of individuals with HD. Very few of these exist however, and even fewer have experience with individuals with juvenile or early-onset HD who require special care because of their age and symptoms.

How Can I Help?
In order to conduct HD research, investigators require samples of tissue or blood from families with HD. Access to individuals with HD and their families may be difficult however, because families with HD are often scattered across the country or around the world. A research project may need individuals of a particular age or gender or from a certain geographic area. Some scientists need only statistical data while others may require a sample of blood, urine, or skin from family members. All of these factors complicate the task of finding volunteers. The following NINDS-supported efforts bring together families with HD, voluntary health agencies, and scientists in an effort to advance science and speed a cure.

The NINDS-sponsored HD Research Roster at the Indiana University Medical Center in Indianapolis makes research possible by matching scientists with patient and family volunteers. The first DNA bank was established through the roster. Although the gene has already been located, DNA from individuals who have HD is still of great interest to investigators. Of continuing interest are twins, unaffected individuals who have affected offspring, and individuals with two defective HD genes, one from each parent-a very rare occurrence. Participation in the roster and in specific research projects is voluntary and confidential.

What is the Role of Voluntary Organizations?
Private organizations have been a mainstay of support and guidance for at-risk individuals, people with HD, and their families. These organizations vary in size and emphasis, but all are concerned with helping individuals and their families, educating lay and professional audiences about HD, and promoting medical research on the disorder. Some voluntary health agencies support scientific workshops and research and some have newsletters and local chapters throughout the country. These agencies enable families, health professionals, and investigators to exchange information, learn of available services and benefits, and work toward common goals. The organizations listed in the Information Resources section of this brochure welcome inquiries from the public.

Glossary
Akinesia–impaired body movement.
At-risk–a description of a person whose mother or father has HD or has inherited the HD gene and who therefore has a 50–50 chance of inheriting the disorder.
Autosomal dominant disorder–a non-sex-linked disorder that can be inherited even if only one parent passes on the defective gene.
Basal ganglia–a region located at the base of the brain composed of four clusters of neurons, or nerve cells. This area is responsible for body movement and coordination. The neuron groups most prominently and consistently affected by HD—the pallidum and striatum—are located here. See neuron, pallidum, and striatum.
Caudate nuclei–part of the striatum in the basal ganglia. See basal ganglia, striatum.
Chorea–uncontrolled body movements. Chorea is derived from the Greek word for dance.
Chromosomes–the structures in cells that contain genes. They are composed of deoxyribonucleic acid (DNA) and proteins and, under a microscope, appear as rod-like structures. See deoxyribonucleic acid (DNA), gene.
Computed Tomography (CT)–a technique used for diagnosing brain disorders. CT uses a computer to produce a high-quality image of brain structures. These images are called CT scans.
Cortex–part of the brain responsible for thought, perception, and memory. HD affects the basal ganglia and cortex. See basal ganglia.
Deoxyribonucleic Acid (DNA)–the substance of heredity containing the genetic information necessary for cells to divide and produce proteins. DNA carries the code for every inherited characteristic of an organism. See gene.
Dominant–a trait that is apparent even when the gene for that disorder is inherited from only one parent. See autosomal dominant disorder, recessive, gene.
Gene–the basic unit of heredity, composed of a segment of DNA containing the code for a specific trait. See deoxyribonucleic acid (DNA).
Huntingtin–the protein encoded by the gene that carries the HD defect. The repeated CAG sequence in the gene causes an abnormal form of huntingtin to be formed. The function of the normal form of huntingtin is not yet known.
Kindred–a group of related persons, such as a family or clan.
Magnetic Resonance Imaging (MRI)–an imaging technique that uses radiowaves, magnetic fields, and computer analysis to create a picture of body tissues and structures.
Marker–a piece of DNA that lies on the chromosome so close to a gene that the two are inherited together. Like a signpost, markers are used during genetic testing and research to locate the nearby presence of a gene. See chromosome, deoxyribonucleic acid (DNA).
Mitochondria–microscopic, energy-producing bodies within cells that are the cells' "power plants."
Mutation–in genetics, any defect in a gene. See gene.
Myoclonus–a condition in which muscles or portions of muscles contract abnormally.
Neuron–a nerve cell, the basic impulse-conducting unit of the nervous system. Nerve cells communicate with other cells through an electrochemical process called neurotransmission.
Neurotransmitters–special chemicals that transmit nerve impulses from one cell to another.
Pallidum–part of the basal ganglia of the brain. The pallidum is composed of the globus pallidus and the ventral pallidum. See basal ganglia.
Positron Emission Tomography (PET)–a tool used to diagnose brain functions and disorders. PET produces three-dimensional, colored images of chemicals or substances functioning within the body. These images are called PET scans.
Prevalence–the number of cases of a disease that are present in a particular population at a given time.
Putamen–an area of the brain that decreases in size as a result of the damage produced by HD.
Receptor–recognition sites on cells that cause a response in the body when stimulated by certain chemicals called neurotransmitters. They act as on-and-off switches for the next nerve cell. See neuron, neurotransmitters.
Recessive–a trait that is apparent only when the gene or genes for it are inherited from both parents. See dominant, gene.
Senile chorea–a relatively mild and rare disorder found in elderly adults and characterized by choreic movements. It is believed by some scientists to be caused by a different gene mutation than that causing HD.
Striatum–part of the basal ganglia of the brain. The striatum is composed of the caudate nucleus, putamen, and ventral striatum. See basal ganglia, caudate nuclei.
Trait–any genetically determined characteristic. See dominant, gene, recessive.
Transgenic mice–mice that receive injections of foreign genes during the embryonic stage of development. Their cells then follow the "instructions" of the foreign genes, resulting in the development of a certain trait or characteristic. Transgenic mice can serve as an animal model of a certain disease, telling researchers how genes work in specific cells.
Ventricles–cavities within the brain that are filled with cerebrospinal fluid. In HD, tissue loss causes enlargement of the ventricles.