FMR1 and fragile X-associated tremor/ataxia syndrome: Fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder affecting subjects (premutation carriers) with a 55-200 CGG-trinucleotide expansion in the 5′UTR of the fragile X mental retardation 1 gene (FMR1) typically after age 50.