The primary mutation known to cause FXS results from an expansion of a cytosine-guanine-guanine (CGG) trinucleotide repeat sequence in the first exon and promoter of FMR1. The repeat sequence is located in the 5′ untranslated region of the FMR1 mRNA, and thus is not translated and does not affect the sequence or structure of its encoded product, the Fragile X Mental Retardation Protein (FMRP). This evidence concerns the gene FMR1 and fragile X syndrome.