FMR1 and fragile X-associated tremor/ataxia syndrome: In addition to different age of onset and clinical manifestations, FXTAS and FXS have unique molecular pathogenetic features; FXS is related to transcriptional silencing with reduced or absent FMR1 protein, whereas FXTAS is characterized by increased FMR1 mRNA, which is thought to cause cellular injury via a toxic gain-of-function (72).