FMR1 and fragile X syndrome: FXS is caused by a CGG repeat expansion in the 5′UTR region of the FMR1 gene, leading to transcriptional silencing and loss of fragile X messenger ribonucleoprotein (FMRP) from both neurons and glia in the brain (Verkerk et al. 1991; Richter, Bassell, and Klann 2015).