FMR1 and fragile X syndrome: FXS is a single-gene disorder caused by a CGG repeat expansion on the Fragile X Messenger Ribonucleoprotein 1 (FMR1) gene on the X chromosome, resulting in reduced production of Fragile X Messenger Ribonucleoprotein Protein (FMRP), which is a protein that is crucial to brain development [30].