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).
Martin and Bell in 1943, described a pedigree of X-linked mental disability, without considering the macroorchidism (larger testicles). In 1969 Herbert Lubs first sighted an unusual "marker X chromosome" in association with mental disability. 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.
Full article ▸