Fragile X syndrome in mildly mentally retarded children in a northern Swedish county. A prevalence study.

In an extensive etiological study of an unselected series of mildly mentally retarded children (MMR) (IQ 50-70) born 1959-1970 in a northern Swedish county, 5 of 110 boys (4.5%) and none of 61 girls had a fragile site on the distal end of the X-chromosome (Fra Xq 28). Consequently fragile X was seen in 2.9% of the total series of 171 children. In a combined series of severe and mild mental retardation, the incidence of the fragile X syndrome was calculated to be 1:3000 in the county of Västerbotten. Next to trisomy 21 the fragile X syndrome was the most common single identified cause of MMR in boys. A cytogenetic investigation using special cultural conditions and banding techniques should be performed in cases of mental retardation of unclear etiology and in possible female carriers.