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Am J Med Genet. 1991 Feb-Mar;38(2-3):411-5.

X chromosome inactivation in fibroblasts of mentally retarded female carriers of the fragile site Xq27.3: application of the probe M27 beta to evaluate X inactivation status.

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  • 1Murdoch Institute, Royal Children's Hospital, Parkville, Melbourne, Australia.


Over 30% of female carriers of the fragile X [fra(X)] syndrome are clinically affected. A nonrandom X chromosome inactivation in these cases could be a plausible explanation. A review of previous studies addressing this question showed inconclusive results; thus, we analysed the X inactivation pattern in fibroblasts of 4 unrelated, mentally retarded fra(X) carriers with a high expression of the fragile site Xq27.3. Using Southern analysis with a highly polymorphic probe M27 beta that recognizes methylation differences between the active and inactive X chromosome we found a 50/50 inactivation pattern in 2 cases and skewed patterns in the other 2. As biased patterns were also observed in control females we conclude that at present no evidence exists for a nonrandom X chromosome inactivation in the fra(X) syndrome in females.

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