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X-Sense, Fragile X with Reflex

Test code(s) 16313, 16321 (NY)

This is an outdated version of this FAQ. It was effective 05/31/2013 to 02/15/2015.

The current version is available here.

Question 1. What does a “gray zone" allele carrier mean?

Female gray zone (also known as intermediate) allele carriers have one normal size CGG repeat and one that is between 45 and 54 CGG repeats in length. The stability of the allele with 45-54 CGG repeats is unknown. It may be stable from generation to generation or it may undergo expansion in future generations. There have been a few  reports of a gray zone allele expanding to a full mutation within two generations, but the likelihood is low.1

Male gray zone or intermediate allele carriers have one allele that has between 45 and 54 CGG repeats. The stability of this allele is unknown and may be stable when passed to his daughters, or may expand to a premutation, which could have an impact on his grandchildren.

Question 2. What is the next step if my patient tests positive for a gray zone allele?

Genetic counseling is available to discuss the options available for further testing, if necessary or desired by the patient.

Question 3. The report indicates my female patient has two FMR1 alleles in the normal size range (negative). Does this mean my patient is not a fragile X carrier?

The majority of female patients with a negative result (ie, two normal size CGG repeats in the FMR1 gene) are not fragile X carriers. This is true even if the patient has an intellectual disability.  However, there are extremely rare cases in which fragile X syndrome is caused by a mutation other than a CGG expansion.  If other potential causes of the clinical presentation have been ruled out and the patient has classic fragile X features or a fragile X positive family history, please call a genetic counselor at 866-GENEINFO to discuss.   

Question 4. The report indicates my male patient has one FMR1 allele in the normal size range (negative). Does this mean he does not have fragile X syndrome?

A male patient with one FMR1 allele in the normal size range would not be expected to have fragile X syndrome. However, there are extremely rare cases of fragile X syndrome that are due to different types of mutations within the fragile X gene. If other causes for the patient’s clinical presentation have been ruled out and the patient has classic fragile X features, or has a positive family history of fragile X syndrome, please call one of our genetic counselors at 866-GENEINFO to discuss.

Question 5. My patient has a family history of fragile X and has a normal fragile X CGG repeat result. Does this result guarantee that my patient is not a carrier of fragile X?

Since most cases of fragile X are due to the expansion of the CGG repeats tested for in this assay, a negative result rules out carrier status in most cases. We recommend that careful comparison of the report of the specific fragile X mutation in the affected family member is performed and compared to this negative result in your patient to confirm the accuracy of the test result. Please call 866-GENEINFO to discuss this case with a genetic counselor if you have further questions.

 

References

  1. J Mol Diagn, 2009;11: 306-310
This FAQ is provided for informational purposes only and is not intended as medical advice. A physician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.

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