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Fragile X syndrome is an inherited form of learning disability that was defined in the late 1970s by cytogenetic detection of an associated fragile site on the X chromosome (Xq27.3). Cytogenetic estimates of the prevalence of fragile X syndrome were as high as 1 in 1039 males but have since been revised downwards. Fragile X syndrome is associated with few medical problems and the subtle physical features make clinical diagnosis difficult. The unusual pattern of inheritance, delineated in the 1980s, was explained once the fragile X syndrome gene (FMR1) had been identified in 1991. This gene contains a highly variable repeat of the nucleotide triplet, cytosine-guanine-guanine (CGG). Fragile X syndrome is caused by a large expansion of this CGG repeat (full mutation) that leads to silencing of the FMR1 gene so no gene product (FMRP) is made. This is the ultimate cause of the learning disability that, in males, is sufficient to preclude independent living. Family studies show that all individuals with a full mutation inherit it from a female (usually unaffected) who carries either a full mutation or a premutation, a smaller repeat expansion (approximately 55-200 repeats) that is unstable on female transmission. The chance of a premutation expanding to a full mutation is positively associated with the size of the repeat (approximately 95% by 90 repeats) but only for female transmissions. When a man transmits a premutation, it remains a premutation; his children are, therefore, unaffected by overt learning difficulties. The potential for population screening or systematic case-finding and extended family testing exists because every unaffected mother of an affected child has a detectable CGG repeat expansion. Reliable prenatal diagnosis is possible in males.

An assessment of screening strategies for fragile X syndrome in the UK