FMR1 and fragile X-associated tremor/ataxia syndrome: Fragile X-associated tremor/ataxia syndrome (FXTAS) is a severe neurodegenerative movement disorder caused by premutation expansions (55–200 CGG repeats) in the 5′ untranslated region of the fragile X mental retardation 1 (FMR1) gene, located on the X-chromosome.