Martin-Bell syndrome

Alternative names 
Fragile X syndrome; Marker X syndrome

Fragile X syndrome is a genetic condition involving changes in the long arm of the X chromosome. It is characterized by mental retardation.

Causes, incidence, and risk factors
Fragile X syndrome is the most common form of inherited mental retardation in males and a significant cause in females. The inheritance is different from common dominant or recessive inheritance patterns. A fragile area on the X chromosome tends to repeat bits of the genetic code. The more repeats, the more likely there is to be a problem. Boys and girls can both be affected, but because boys have only one X chromosome, a single fragile X is more likely to affect them more severely.


  • Family history of fragile X syndrome, especially a male relative  
  • Mental retardation  
  • Large testicles (macro-orchidism)  
  • Large size  
  • Tendency to avoid eye contact  
  • Hyperactive behavior  
  • Large forehead and/or ears with a prominent jaw

Signs and tests
A specific genetic test (PCR) can now be performed to diagnose this disease. This test looks for an expanded mutation (called a triplet repeat) in the FRAXA gene. Formerly a specific type of chromosome analysis was done and this may still be available.

There are very few outward signs of Fragile X syndrome in babies but one is a tendency to have large head circumference. Measurement of oversized testes in males can also suggest the diagnosis. An experienced geneticist may note subtle differences in facial characteristics. Mental retardation is the hallmark of this condition and, in females, this may be the only sign of the problem.

There is no specific treatment for Fragile X syndrome. Instead, effort is directed toward training and education so that affected children can function at as high a level as is possible. Because the condition is not rare, specific educational approaches have been developed and tested.

Support Groups
National Fragile X Foundation

Expectations (prognosis)
The outcome depends on the extent of mental retardation.

Complications vary depending on the type and severity of symptoms.

Calling your health care provider
Call your health care provider if you suspect the possibility of Fragile X syndrome in a child with mental retardation.

Genetic counseling may help prospective parents with a family history of Fragile X syndrome. Genetic testing can help determine the level of risk in such a family.

Johns Hopkins patient information

Last revised: December 6, 2012
by Simon D. Mitin, M.D.

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