The most severe premutation-associated disorder affecting carriers of the FMR1 premutation allele is the late onset progressive neurodegenerative condition termed Fragile X Associated Tremor/Ataxia Syndrome (FXTAS), affecting 40−50% of male carriers after the age of 55, and 8–16.5% female carriers in the same age group (2–4). The gene discussed is FMR1; the disease is fragile X-associated tremor/ataxia syndrome.