The expansion triggers an epigenetic mechanism of DNA methylation and additional chromatin remodelling culminating in the transcriptional silencing of FMR1. Interestingly, when the repeat is present in the “premutation range” of 55 to 200 units, carriers do not develop fragile X syndrome but are at risk for the late-onset neurodegenerative disorder, FXTAS. This evidence concerns the gene FMR1 and fragile X syndrome.