See Also

Fragile X Chromosome

Disease Characteristic
Fragile X syndrome (FXS), the most common cause of inherited mental impairment. This impairment can range from learning disabilities to more severe cognitive or intellectual disabilities. (Sometimes referred to as mental retardation.) FXS is the most common known cause of autism or "autistic-like" behaviors. Symptoms also can include characteristic physical and behavioral features and delays in speech and language development.

How Common is Fragile X Mutation?
Approximately 1 in 800 men in the world are carriers of the Fragile X permutation. Approximately 1 in 260 women in the world are carriers of the Fragile X permutation. Though a recent 2004 analysis suggests that the figure could be as high as 1 in 130.    

Based on this screening scheme, the prevalence of the full mutation in Caucasian populations is approximately 1 in 4,000 males to 1 in 6,000 males, with point estimates ranging from 1 in 3,717 males to 1 in 8,918 males.

Genes and Mutations (Genotype-Phenotype Relationships)
A person with fragile X syndrome has a mutation in the FMR1 (fragile X mental retardation 1) gene in the DNA that makes up the X chromosome. That mutation causes the cell to methylate a regulatory region of the FMR1 gene. The methylation turns off the FMR1 gene.

Mutations and populations    
For premutations (61-200 repeats), the estimates for Caucasian females range from 1 in 246 to 1 in 468 in the general population. For Caucasian males, the prevalence of the premutation (61-200 repeats) is probably between 1 in 1,000 to 1 in 2,000 in the general population.

Most common form of inherited mental retardation.
A wide range of cognitive, behavioral, and physical features such as variable IQ (profound to mild mental retardation), autistic-like features, hyperactivity, increased testicular volume, macrocephaly, and large ears.

Criteria for Genetic Testing
Prenatal screening is strongly recommended                                                                                                                                        Any person with unexplained mental retardation, developmental delay, or autism.

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