Fragile X syndrome

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Fragile X syndrome (FXS), or Martin-Bell syndrome, is a genetic syndrome which results in a spectrum of characteristic physical and intellectual limitations and emotional and behavioral features which range from severe to mild in manifestation.

The syndrome is associated with the expansion of a single trinucleotide gene sequence (CGG) on the X chromosome, and results in a failure to express the protein coded by the FMR1 gene, which is required for normal neural development. There are four generally accepted states of the chromosome region involved in Fragile X syndrome which relate to the length of the repeated CGG sequence; Normal (29-31 CGG repeats) (not affected by the syndrome), Premutation (55-200 CGG repeats)(not affected by the syndrome), Full Mutation (more than 200 CGG repeats)(affected), and Intermediate or Gray Zone Alleles (40 - 60 repeats).[1]

Martin and Bell in 1943, described a pedigree of X-linked mental disability, without considering the macroorchidism (larger testicles).[2] In 1969 Herbert Lubs first sighted an unusual "marker X chromosome" in association with mental disability.[3] In 1970 Frederick Hecht coined the term "fragile site".

Renpenning's syndrome is not synonymous with the syndrome. In Renpenning's syndrome, there is no fragile site on the X chromosome. Renpenning's cases have short stature, moderate microcephaly, and neurological (brain) disorders.

Escalante's syndrome is synonymous with the fragile X syndrome. This term has been used in South American countries.

Fragile X is the most common known single gene cause of autism and the most common inherited cause of intellectual disability.[4][5]

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