Fragile X syndrome (FXS) is a genetic condition that produces a number of different birth defects. It can cause cognitive impairment (e.g., mental retardation), emotional and behavioral problems (e.g., attention-deficit hyperactivity disorder), as well as distinctive physical characteristics (e.g., prominent ears, elongated face, enlarged testicles after puberty).

FXS occurs due to a genetic stutter on the fragile X mental retardation 1 (FMR1) gene. This gene is located on the X chromosome, which is one of two sex-defining chromosomes that children inherit from their parents.

A section of DNA in the FMR1 gene includes a certain sequence of nucleotides (building blocks of DNA): cytosine, guanine and guanine (CGG). Normally, this sequence repeats approximately 30 times. When the CGG sequence is repeated too many times (more than 200 times), a certain protein (fragile X mental retardation protein, FMRP) fails to be made. A lack of this protein triggers FXS. It is not clear exactly why this happens, although FMRP appears to affect communication between nerve cells in the brain.

FXS involves an abnormality in a single gene on a particular chromosome. Thus, it is different from Down syndrome, in which an extra chromosome is present. It is called “fragile” X syndrome because when the X chromosome is viewed under a microscope, the area with the mutation looks as if it is barely connected to the rest of the chromosome.

FXS affects males more severely than females because the gene mutation is located on the X chromosome. Males have only one X chromosome, which they inherit from their mother, whereas females have two (one from each parent). Females that inherit an X chromosome with the mutated gene may offset its effects by having a second chromosome with a normal (non-mutated) FMR1 gene. Thus, females with FXS are more likely to have less severe symptoms than males with the syndrome.

FXS occurs in about one in every 4,000 male infants and around one in every 8,000 female infants, according to the National Institutes of Health. It is the most common cause of mental retardation in boys, and it is a significant cause of mental retardation in girls. FXS can also cause autism in affected children.

Approximately 20 percent of children with FXS develop seizures, according to the March of Dimes. Children with FXS may be at increased risk of sinusitis, otitis media, vision problems and digestive disorders.