Long trinucleotide (CGG) expansion (i.e., full mutations) in the 5′ UTR of the FMR1 gene are associated with the neurodevelopmental disorder fragile X syndrome (FXS), while short, “premutations” are associated with fragile X-associated tremor and ataxia syndrome (FXTAS), a late-onset condition characterized by executive functioning decline and progressive cerebellar ataxia, presenting in a subset of premutation carriers (3, , , –7). Here, FMR1 is linked to fragile X-associated tremor/ataxia syndrome.