Prenatal Diagnosis of the Fragile-X Syndrome


The Martin-Bell syndrome is a common form of X-linked mental retardation, characterised by co-segregation with a fragile site at Xq27.3 (FRAXA). With a birth frequency of approximately 0.4–0.8/1000 males and 0.2–0.6/1000 females [1–4], this syndrome is second only to Down’s as the most common form of heritable mental retardation. Penetrance in males in whom the fragile-X can be demonstrated (as opposed to the usually FRAXA negative yet gene-carrying transmitting males) is considered to be approximately 100%, while about one-third of carrier females are mentally impaired to some degree. The fragile site is nearly always found in affected female heterozygotes but is detected in only between a half and two-thirds of those of normal intelligence. In general those who are handicapped express the fragile Xq27.3 more easily than those who are not, but approximately 10% of normal female carriers do express FRAXA at a high level [5,6]


Amniotic Fluid Prenatal Diagnosis Fragile Site Fetal Blood Fetal Abnormality 
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© Springer-Verlag London Limited 1991

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  • T. Webb

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