FMR1 and fragile X syndrome: In almost all cases, FXS is caused by a CGG repeat expansion in the 5′ untranslated region of the fragile X mental retardation 1 gene (FMR1) and hypermethylation of its promoter region on the X chromosome, which results in FMR1 transcriptional silencing and absence of the fragile X mental retardation protein (FMRP) encoded by this gene (Santoro et al., 2012).