Fragile X syndrome (FXS) is a single gene disorder caused by a large (>200) CGG repeat expansion in the promoter region of the Fragile X Messenger Ribonucleoprotein 1 (FMR1) gene on the X chromosome, which leads to excessive methylation of the gene, transcriptional silencing, and loss or reduced production of the Fragile X Messenger Ribonucleoprotein Protein (FMRP). This evidence concerns the gene FMR1 and fragile X syndrome.