The most severe form of clinical disorder associated with premutation expansions in the 5′ untranslated region of the Fragile X Mental Retardation 1 (FMR1) gene is a late-onset progressive neurodegenerative condition: Fragile X-Associated Tremor/Ataxia Syndrome (2). Here, FMR1 is linked to fragile X-associated tremor/ataxia syndrome.