ABSTRACT
The fragile X syndrome was the most frequent etiology of hereditary mental retardation but the clinical diagnosis is not easy and the indivivual clinical symptoms were not specific so the confirmation will be made par molecular study of the gene of the fragile X syndrome. of our study is to realise the molecular diagnosis of the fragile X syndrome in 200 Tunisian boys with mental retardation. Shows that the frequency of the fragile X syndrome is 7.6%. In the most cases there is a family history of mental retardation with midium age at 11 years. All the boys with the full mutation have mental retardation, dysmorphic features and macro-orchidism [pubescent boy]. The screening of the molecular abnormalitie of FMRI gene must be realised in every boy with mental retardation or boy with delayed speach without any identified etliology. The earlier diagosis is important-for genetic counselling