FRAGILE-X-SYNDROME IS LESS COMMON THAN PREVIOUSLY ESTIMATED

In 1986, a population study of school children in the city of Coventry gave an overall prevalence in males and females for fragile X syndrom e of 1/952. The 29 children diagnosed as having fragile X syndrome in this study have been re-evaluated with molecular diagnostic techniques . Eighteen of the original 29 children have been found not to have the expansion of the FMR1 gene associated with fragile X syndrome. Revise d prevalence figures have been calculated giving rise to an overall pr evalence figure of 1/2720 (range 1/2198-1/3089). If the four children lost to follow up are also assumed not to have the fragile X syndrome, the revised prevalence figure was 1/5714 (range 1/4762-1/6349). Clini cal review of boys with severe mental retardation from this and a subs idiary study show that the clinical features of head circumference gre ater than the 50th centile, testicular volume greater than the 50th ce ntile, and IQ between 35 and 70 remain helpful in distinguishing boys with fragile X syndrome from those who have nonspecific mental retarda tion.