What is fragile X syndrome?
Fragile X syndrome is the most common form of inherited
mental retardation in males and is also a significant cause of
mental retardation in females. It affects about 1 in 4,000
males and 1 in 8,000 females and occurs in all racial and
ethnic groups.
Nearly all cases of fragile X syndrome are caused by an
alteration (mutation) in the FMR1 gene where a DNA segment,
known as the CGG triplet repeat, is expanded. Normally, this
DNA segment is repeated from 5 to about 40 times. In people
with fragile X syndrome, however, the CGG segment is repeated
more than 200 times. The abnormally expanded CGG segment
inactivates (silences) the FMR1 gene, which prevents the gene
from producing a protein called fragile X mental retardation
protein. Loss of this protein leads to the signs and symptoms
of fragile X syndrome. Both boys and girls can be affected, but
because boys have only one X chromosome, a single fragile X is
likely to affect them more severely.
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