Expansion of repetitions of the trinucleotide sequence of cytosine and guanine (CGG) on the FMR1 gene is the cause of several well-described disorders including fragile X syndrome (FXS), fragile X tremor ataxia syndrome (FXTAS), and fragile X primary ovarian insufficiency (FXPOI). This evidence concerns the gene FMR1 and fragile X-associated tremor/ataxia syndrome.