In most cases FXS is caused by transcriptional silencing of the FMR1 gene and the ensuing lack of encoded Fragile X Mental Retardation Protein (FMRP) (reviewed in ref. 2), an RNA-binding protein that regulates translation and trafficking of its interacting mRNAs in dendrites and axons (reviewed in ref. 3). The gene discussed is FMR1; the disease is fragile X syndrome.