The science and practice of testing for Huntington's disease
What does the actual test entail?
The genetic test itself is simply a blood test, in which a small sample of blood is drawn from the individual being tested. Technically, it is unnecessary to obtain blood samples from other family members , (except in the case of the non-disclosing prenatal test discussed in Part 4). However, testing centers may request a blood sample, genetic test results, or the medical records of a family member who has HD in order to confirm the diagnosis of HD in the family. In prenatal testing, DNA samples are obtained from the fetus by chorionic villus sampling (CVS) or amniocentesis.
The blood sample is then sent to a laboratory where DNA testing is performed to check for the expanded CAG repeat (To learn more about expansions and mutations, click here.) within the Huntington gene. DNA is isolated from the blood sample and amplified through the polymerase chain reaction (PCR), a technique for rapidly producing millions of copies of a particular stretch of DNA. In this case, PCR is used to amplify the region of DNA containing the Huntington gene, allowing for closer study.
The size of the CAG repeat can then be assessed through gel electrophoresis. First, restriction enzymes cleave the DNA into even smaller segments of various sizes. These fragments are placed in a porous gel that floats in a salty buffer solution, in a chamber between two electrodes. An electric current is passed through the chamber, causing the negatively charged DNA molecules to begin migrating through the gel, toward the positively charged electrode. The DNA segments become separated throughout the gel based on their size, as smaller fragments move through the gel “sieve” faster than larger fragments. Then, the separated DNA fragments can be examined individually and “counted,” indicating the number of CAG repeats in the Huntington gene. This number determines whether or not the individual will develop HD.
Last Modified: 9-13-02
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The size of the CAG repeat can then be assessed through 
