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BACKGROUND:Fragile X syndrome (FXS),an X-linked disorder,is the most common cause of inherited mental retardation.This is caused by a trinucleotide CGG repeat expansion (＞ 200) on the fragile X mental retardation 1 gene (FMR1) becoming methylated leading to a deficiency or absence of the FMR1 protein.Determining FXS prevalence in the mentally retarded individuals in the west of Iran was the aim of this study.METHODS:200 patients with moderate mental retardation who were clinically suspicious to FXS were screened using cytogenetic and molecular methods.Blood samples were collected and cultured in the specific culture media.The G-Banding method was used for karyotyping and DNA sequencing performed for verifying the results of the cytogenetic tests.RESULTS:16 patients (8％) were found to have fragile X syndrome.The results showed that there is no significant association between the fragile X syndrome and economic status and place of residence,however,the relationship between fragile X syndrome and mental retardation in the family history is significant.CONCLUSION:The frequency of FXS was similar to other reports in the preselected patients.For diagnosis of FXS,chromosome analysis must be accompanied by molecular studies.