Fragile X syndrome (FXS) is now the most common known inherited cause of developmental disabilities, but was not discovered until the late 1970s. By 1980 it was found that people showing certain mental and physical characteristics had a chromosomal abnormality caused by a partial break on an X chromosome, called a "fragile site". In 1991 the Fragile X gene was identified within this site.
FXS is named after a site on the long arm of the X chromosome that is elongated and appears partly broken or 'fragile'. The spectrum of Fragile X syndrome ranges from normal development to developmental delay, learning disabilities, mild to severe intellectual disability, autistic-like behaviour and attentional problems. The majority of children are mildly to moderately affected. There may or may not be other known affected family members. Genetic testing has changed the approach to the diagnosis of the condition because there is now a reliable and relatively simple blood detection test which is available to all children with developmental delay of unknown cause.
The diagnosis of FXS is important to implement effective management and interventional strategies. Detection of carriers allows families to make informed decisions following genetic counselling.
Estimates suggest about 1 in 4,000 males are affected and that about 1 in 1,000 females carry the gene. Many affected family members are unaware of the genetic cause and have yet to be diagnosed. In comparison, 1 in 700 live births has Down's syndrome (which is not usually inherited).
The Fragile X Syndrome
The clinical features of Fragile X syndrome include physical, developmental and behavioural characteristics and range from normal through mild to severe in presentation. Intellectual disability (IQ less than 70) is present in 80% of males and 50% of females. Children may have been labelled as having pervasive developmental disorder,