FMR1 and fragile X syndrome: Overall, these disorders, known as FRAXopathies [8] or fragile X-related disorders [9], share the CGG triplet instability of the FMR1 gene but are characterized by opposed epigenetic changes, with decreased transcription in FXS patients (loss-of-function) and increased transcription in PM carriers, resulting in a toxic gain-of-function of the excess mRNA [10].