Prevalence of the fragile‐X syndrome in mentally retarded boys in a Swedish county

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Abstract
In an etiological study of an unselected series of mentally retarded children (IQ < 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.