AR and fragile X syndrome: In 1991, two different research groups successively discovered a CAG expansion in the exon of the androgen receptor gene in X-linked spinal and bulbar muscular atrophy (SBMA) [4] and a CGG expansion in the 5’ untranslated region of the fragile X mental retardation 1 (FMR1) gene in fragile X syndrome (FXS) [5].