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Fragile X Syndrome

Fragile X syndrome is the most common cause of inherited mental retardation. The full mutation appears in approximately 1 in 2000 males and 1 in 4000 females. The premutation occurs in about 1 in 250 females and 1 in 700 males. Those who carry the premutation have a high chance of passing either the full mutation or the premutation to their offspring.


The majority of males with fragile X syndrome have a significant intellectual disability. The spectrum ranges from subtle learning disabilities to severe mental retardation and autism. Behavioral characteristics include attention deficit disorders, speech disturbances, hand biting, hand flapping, autistic behaviors, poor eye contact, and aversion to touch and noise.

While some females with fragile X syndrome will exhibit these characteristics, fewer females are affected and their degree of impact is usually diminished.

Because fragile X presents itself in such a myriad of ways, it is often challenging for families to get a clear diagnosis of the disorder. In fact, it is thought that 80 - 90% of those affected with fragile X are undiagnosed or misdiagnosed.

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. Since the gene is turned off, the person doesn't make fragile X mental retardation protein (FMRP). The lack of that protein triggers fragile X syndrome.

Females are less affected than males because females have two X chromosomes (males have one X and one Y). Males with fragile X only get the nonfunctioning gene while females with fragile X get one normally functioning gene to partially compensate for the nonfunctioning gene.

Men with the premutation will pass the altered gene to all of their daughters and none of their sons. Women with the premutation have a 50% chance of passing the altered gene to all of their offspring.

Special education, speech and language therapy, occupational therapy and behavioral therapies help to address many of the physical, behavioral, and cognitive impacts of fragile X syndrome. Medical intervention can help with aggression, anxiety, hyperactivity and poor attention span. Currently, there is no cure for fragile X syndrome.

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