Two families with Xq27.3 fragility, no detectable insert in the FMR‐1 gene, mild mental impairment, and absence of the Martin‐Bell phenotype

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Abstract
In 2 families, propositi were investigated because of mild developmental delay and, in one case, behavior disorders. Seven males in the 2 families were found to have a fragile site at Xq27.3 but the usual insert in the FMR‐1 gene was absent. The affected males had mild, or in some cases, no clear intellectual impairment and did not have the Martin‐Bell phenotype. Carrier females in one family tended to show a high level of cytogenetic expression of the fragile site but were clinically normal. It is not yet clear whether these families have unusual mutations in the FMR‐1 gene or whether their fragile sites are different, but cytogenetically indistinguishable from, that associated with inserts in the FMR‐1 gene.