Résumé: Fragile X syndrome (FXS) is the leading cause of inherited mental retardation. The underlying molecular alteration consists of a CGG-repeat expansion within the FMR1 gene. Therefore, searching for CGG expansion at the FXS locus among the mentally retardedshould become a routine investigation in neuro-paediatric practice. This study was to establish a molecular diagnosis in Algerianpatients exhibiting mental retardation with or without autistic features. It is important for the fragile X diagnosis to establishwhether the range of allele distribution in Algerian patients are comparable to other populations, and if it is observed the samepattern of expansion associated with the disease. PCR was undertaken on 60 samples followed by Southern blot to analyze the CGGrepeat number and methylation status. The molecular findings indicated in one young patient, diagnosed as autistic, mosaicsmutationsof 360 and 330 repetitions, while his mother was found carrying a premutated allele of 87 repetitions, and his two yearsold sister was found to have a contracted premutated allele of 63 repetitions of the CGG repeat. The distribution of the CGG repeatsfalls within the 30 repeats (17%), followed by a 31 repeat sizes (12%) instead of the 29 repeats as in the Caucasian population. Theresults of this study reconfirmed previous reports that the pattern of FMR1 CGG repeat alleles is different regarding theracial/ethnical population studied. In conclusion, the detection of the FXS mutations has allowed us to offer more informed geneticcounseling and reliable patient follow-up.