FXS patients also display considerable clinical and genetic heterogeneity, which manifests in a wide spectrum of behavioural phenotypes among patients.8 This heterogeneity is thought to stem from the heterogeneous genetic background of patients as well as the existence of mosaicism of FMR1 methylation, which results in a differential expression of FMR1 across the brain.9 It is therefore not surprising that despite several potential targets being uncovered and trialled in the clinic,10 no disease-modifying therapy currently exists that is able to address the multiple symptoms in FXS patients. This evidence concerns the gene FMR1 and fragile X syndrome.