The first genetic expansions (identified in 1991) consisted of CGG repeats in the 5′untranslated region of the fragile X messenger ribonucleoprotein 1 (FMR1) gene, associated with fragile X syndrome (FXS), and CAG repeats in the first exon of the androgen receptor (AR) gene, linked to spinal and bulbar muscular atrophy (SBMA). This evidence concerns the gene FMR1 and fragile X syndrome.