Huntington Disease: CAG Repeat Analysis

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A signed consent form for Huntington disease testing is mandatory.

Condition Description

Huntington disease is a progressive neurodegenerative disease characterized by chorea and dementia. This disease is inherited in an autosomal dominant fashion and affects approximately 1 per 10,000 individuals. Although patients may develop symptoms at any time from childhood to later in life, the average age of onset is between 35 and 45. With paternal transmission of the gene, there is a tendency for earlier onset to occur. Over 80% of the patients with an onset before the age of 20 inherited the abnormal gene from their father. Mutations may occur spontaneously as well, but this is uncommon.

Huntington disease is caused by an abnormal expansion of a CAG repeat within the HTT/ITI5 on chromosome 4. Although the number of CAG repeats correlates with the age of onset, the correlation is not precise enough to use in determining disease prognosis for an individual patient.

Due to the complexities and ethical issues surrounding genetic testing for Huntington, a signed consent form for Huntington disease testing is mandatory. Please call our laboratory to review your patient's case in advance and obtain the necessary forms. 

Click here for the GeneReviews summary on this condition.

Genes (3)

Your search may have returned a result for a gene alias. Click here for a full list of genes and their aliases.


This test is indicated for:

  • Confirmation of a clinical diagnosis of Huntington disease
  • Presymptomatic testing in adults with a family history of Huntington disease


CAG repeat number is determined by PCR amplification and fragment size analysis.

Detection and Reference Range

Nearly 100% of CAG expansions in the HTT/ITI5 gene will be detected by this assay.
Reference Range
Normal individuals have 6-35 repeats while individuals with greater than 39 CAG repeats have a high probability of developing the disease. When the repeat number is between 36 and 39, Huntington disease cannot be reliably diagnosed due to incomplete penetrance.

Specimen Requirements

When sample fails to meet the acceptable criteria, please call 470.378.2200 and ask to speak with a laboratory genetic counselor (

Type: Whole Blood

Specimen Requirements:

In EDTA (purple top) or ACD (yellow top) tube:
Infants (Children (>2 years): 3-5 ml
Older Children & Adults: 5-10 ml

Specimen Collection and Shipping: Refrigerate until time of shipment. Ship sample within 5 days of collection at room temperature with overnight delivery.

Special Instructions

A signed consent form for Huntington disease testing is mandatory.

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