FMR1 and fragile X syndrome: The prevalence of Fragile X syndrome in the general population is ~1 in 4000.2 The more common FMR1 premutation (PM) expansion (55–199 CGG repeats), which is found in ~1 in 209 females and 1 in 430 males,3 confers the risk of developing Fragile X-associated tremor/ataxia syndrome (FXTAS).