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What should I do if I find out someone in my family has fragile X syndrome?

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If someone in your family, a child or an adult, is diagnosed with Fragile X, you may also want to be tested to see if you have a mutated FMR1 gene. It is now possible to test for Fragile X in people of any age, as well as before birth. These tests are simple and accurate.

At present, testing for Fragile X is not done routinely. The tests are often done to help diagnose a child who is developmentally delayed or shows signs of autism or mental retardation. 19 Couples who have one or more relatives with mental retardation of unknown cause may also want to be evaluated before deciding to have a child.

Health care professionals may also recommend an evaluation for Fragile X in a person with one of the following traits: 20

  • Any person who has mental retardation of unknown cause, developmental delay, or learning disability
  • Any person with autism or showing autistic-like behaviors
  • Any person with a relative who has Fragile X or mental retardation of unknown cause
  • Anyone who was previously assessed for Fragile X using the chromosome test (see description below)
  • Women with premature ovarian failure (POF) or with a family history of POF

Tests commonly used to diagnose Fragile X

A number of tests are used to diagnose Fragile X. Each test has its limitations, so in many cases more than one test may be used. The most accurate tests are DNA tests.

  • DNA molecular tests count the number of CGG repeats in the promoter region of the FMR1 gene and check to see if the gene is methylated (inactive). These tests are used most often to diagnose Fragile X. The two main DNA tests include:
    • Southern Blot studies how quickly DNA pieces move through a liquid or gel when exposed to electricity. Although good at finding large numbers of repeats and determining methylation, this process is rather slow.
    • Polymerase Chain Reaction (PCR) multiplies the amount of DNA being tested so it can be analyzed more quickly. Although it can miss some large repeat mutations, this test is much faster to perform and is very good at detecting small and medium-sized repeat segments.
  • Chromosome test looks for the “broken” area of a chromosome using a microscope. Because this test is older, costly, and often inaccurate, it is recommended that another type of test be used to diagnose Fragile X. The DNA test is needed to confirm the diagnosis.
  • Protein test measures the amount of FMRP produced by living cells. This test is useful for screening large groups of people for Fragile X. In the future, this test may be helpful in evaluating how severe an FMR1 mutation is. The DNA test is needed to confirm the diagnosis.

Prenatal testing for families who are at risk

A pregnant woman who has characteristics that place her at risk for Fragile X, such as features of Fragile X or a family history that suggests the presence of Fragile X, may want to have a prenatal test. During such a test, the doctor either takes a sample of amniotic fluid, a process called amniocentesis, or takes a sample of cells from the placenta, a process called chorionic villus sampling. Then the cells are sent to a lab, where they go through one of the methods used to diagnose Fragile X. Cells that contain a larger number of CGG repeats would be diagnosed as having Fragile X.

These tests can be done on many different types of body tissues. In most cases, a simple blood test provides the cells needed. But cells from hair roots and from inside the cheek can also be used. If you or one of your family members wants to be tested, talk to your health care provider. He or she will likely take a sample of your blood and send it to a lab. The results will come back in a number of weeks. For more information on tests for Fragile X and their features, please refer to the Additional Resources section of this booklet.

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Last Updated Date: 08/18/2006
Last Reviewed Date: 08/18/2006
Vision National Institutes of Health Home BOND National Institues of Health Home Home Storz Lab: Section on Environmental Gene Regulation Home Machner Lab: Unit on Microbial Pathogenesis Home Division of Intramural Population Health Research Home Bonifacino Lab: Section on Intracellular Protein Trafficking Home Lilly Lab: Section on Gamete Development Home Lippincott-Schwartz Lab: Section on Organelle Biology