Fragile X Syndrome Info

Fragile X syndrome is the most common inherited form of mental retardation.  It is an X-linked dominant condition that is associated with an expansion of a trinucleotide repeat sequence (CGG)n located on chromosome Xq27.3.  This expansion results in a failure to express the FMR1 gene product, which is required for normal neural development.  In individuals with Fragile X syndrome, the FMR1 allele contains over 200 copies of the (CGG) trinucleotide repeat sequence.  Risk for Fragile X syndrome is determined by the length of the repetitive (CGG) sequence;

  • Normal (<40 CGG repeats)
  • Intermediate or Gray Zone (41 – 60 CGG repeats)
  • Premutation (61-200 CGG repeats)
  • Full Mutation (more than 200 CGG repeats)


  • Affects approximately 1 in 3,600 males and approximately 1 in 4,000-6,000 females
  • It is estimated that 1 in 250 females and 1 in 800 males are premutation carriers
  • Fragile X syndrome is an X-linked dominant condition
  • Women with ovarian failure or an elevated follicle-stimulating hormone level before 40 years of age without a known cause should be screened to determine whether they have the fragile X premutation

Major Phenotypic features

  • Childhood onset
  • Mental deficiency
  • Dysmorphic  facies
  • Male maro-orchidism
  • Affected females present with variable phenotypic features due to random X inactivation

Facial Features

  • Elongated face
  • Large or protruding ears
  • Prominent jaw and forehead

Clinical Management
Currently, medical treatment for Fragile X syndrome is not available.  Patients may benefit from behavioral therapy, special education, medication, and treatment of physical abnormalities.