How is Fragile X syndrome diagnosed?

The FMR1 DNA Test

Fragile X is diagnosed through a DNA sample, usually from blood, but amniotic fluid and other tissues can also be tested.

The FMR1 DNA test can be administered with two different lab procedures:

  • The Southern blot analysis test determines if the gene has a full mutation, its approximate size, whether the gene has been “methylated,” and if there is mosaicism of the gene (a mixture of different cell types).
  • The polymerase chain reaction (PCR) analysis can determine the actual number of CGG repeats (a pattern of DNA) that are present in the Fragile X gene. For various technical reasons, PCR has not been the test of choice to diagnose a full mutation, but is quite accurate in determining premutation and normal gene repeat numbers. However, PCR is less expensive and quicker than Southern blot, and recent advances in technology have increased its ability to identify Fragile X full mutations. PCR may thus be the only test used in the near future.

Prenatal diagnosis refers to testing the fetus early in pregnancy. There are two different methods to evaluate the genetic status of the fetus: amniocentesis and chorionic villus sampling (CVS).

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