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An amplification of a highly unstable DNA element has been identified at the fragile X locus in Xq27.3. This sequence appears to be both the source of the primary mutation causing the fragile X syndrome, apparently having its causative effect through the methylation of the FMR-1 HTF island and the region of cytogenetic fragility. The direct analysis of the genotype of carrier and affected individuals can be used as a direct diagnosis tool which will improve both the accuracy and speed of diagnosis. The identification of hereditary unstable DNA in a disease with such a wide level of non-penetrance and variable phenotype may give clues as to the basis of non-penetrance in other human genetic disorders.

Original publication




Journal article


Clin Sci (Lond)

Publication Date





255 - 264


Chromosome Fragility, DNA, Female, Fragile X Syndrome, Gene Amplification, Genetic Carrier Screening, Genetic Linkage, Genetic Markers, Humans, Male, Mutation