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Genet Med. 2007 Jan;9(1):46-51.

Molecular testing for Fragile X Syndrome: lessons learned from 119,232 tests performed in a clinical laboratory.

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  • 1Nichols Institute, Quest Diagnostics, 33608 Ortega Highway, San Juan Capistrano, California 92690, USA. charles.m.strom@questdiagnostics.com



To examine the data from over 119,000 Fragile X Syndrome tests and 307 prenatal tests to detect unsuspected findings and obtain clinical data when indicated to optimize genetic counseling.


A proprietary database containing 119,232 consecutive postnatal and 307 prenatal FXS tests performed between November 2, 1992 and June 1, 2006 was queried.


The distribution of normal FMR1 alleles was a bimodal distribution with a major peak at 30 repeats and a minor peak at 21 repeats. Of 59,707 tests performed for males, 1.4% had a fully expanded and methylated FMR1 allele. Of 59,525 tests performed for females, 0.61% had an affected FMR1 allele, and 1.7% had a premutation FMR1 allele for a total carrier frequency of 1.3%. When fetuses inherited an expanded maternal allele, the risk of expansion to a full affected allele was 0%, 5%, 30% and 100% for allele sizes of <50, 50-75, 76-100 and >100 repeats, respectively.


These figures can be used for genetic counseling of patients presenting for carrier detection and prenatal diagnosis for Fragile X Syndrome.

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