This, together with the elevated expression of expanded FMR1 mRNA in premutation carriers (Tassone et al., 2000a), gave rise to the hypothesis that a toxic mRNA gain-of-function effect might be responsible for the development of FXTAS (Tassone et al., 2000a,b; Hagerman et al., 2001; Greco et al., 2002). Here, FMR1 is linked to fragile X-associated tremor/ataxia syndrome.