FMR1 and fragile X-associated tremor/ataxia syndrome: Fragile X-associated tremor/ataxia-associated syndrome (FXTAS) is an adult-onset neurodegenerative disorder caused by premutation (PM) range expansion (50–200 repeats) of the trinucleotide (CGG) repeat element in the 5′ untranslated region of the fragile X mental retardation 1 (FMR1) gene.