In an etiological study of an unselected series of mentally retarded children (IQ less than 70) born 1959-1970 in a northern Swedish county, 12 of 205 boys (5.9%) were found to have a fragile site on the distal end of the C-chromosome (fra (X) (q27]. The incidence of the fra (X) syndrome was calculated to be 1:1500 boys in this county. If this is true for the whole of Sweden, 30-40 new cases of the fra (X) syndrome should be born yearly in Sweden. This must be considered a minimum figure, since a certain proportion of individuals with fra (X) are not observed in groups of mentally retarded patients. Next to trisomy 21, the fragile X syndrome is the most common specific cause of mental retardation among mentally retarded boys in Sweden.
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Gustavson et al. (1986) studied this question.
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