Objective To screen the fragilex mental retardation 1 (FMR1) gene mutations and explore the frequency of FMR1 gene mutation in the population with mental retardation in South China.Methods Seventy-two patients (65 males and 7 females) with suspected fragile X syndrome (FXS) in South China were enrolled in our hospitals from October 2009 to April 2014.The CGG trinucleotide repeats in 5'UTR of FMR1 gene and CCG trinucleotide repeats in FMR2 gene were screened respectively by PCR.Southern blotting and capillary electrophoresis sequencing were performed in male patients without normal target bands and suspected female patients;patients with normal CGG alleles were,then,performed exons and 3'-UTR ofFMR1 gene amplification and sequencing.The frequency of FMR1 gene mutation in patients with mental retardation in different countries and regions was compared with statistical analysis.Results Six pedigrees with full mutation (one female and five males being the probands),one pedigree (mother and son) with FMR1 gene deletion and one pedigree (mother and son) with mutation in the transition region were identified in 72 patients with mental retardation.The prevalence of total mutation was 9.7％ (7/72) and that in male patients was 9.2％ (6/65).These results showed significant differences in prevalence as compared with the results from different countries and areas (P＜0.05);there were no variations in 3'UTR ofFMR1 gene and FMR2 gene mutation in the patients with FXS-like phenotype.Conclusions FMR1 mutation frequency may be higher in mental retardation population in southem China as compared with that in developed countries or areas.Targeted screening on the unexplained mental retardation pedigrees (family history) can improve the diagnosis of FXS.Importantly,deletion mutations screening should also be performed in suspected FXS subjects with normal CGG repeats.