Fragile X syndrome

Definition:

A genetic condition caused by spontaneous breaks in the long arm of the X chromosome.

Causes, incidence, and risk factors:

Fragility of the long arm of the X chromosome is thought to be a result of increased methylation at the site of the break. The Fragile X accounts for approximately one-third of all X-linked mental retardation in males and one-tenth of that in females. This syndrome occurs in approximately 1 out of 2,000 males and 1 out of 1,000 females.