FMR1 and fragile X syndrome: FXS is an X-linked disorder caused by large expansions in fragile X messenger ribonucleoprotein-1 (FMR1, >200 CGG repeats), termed the “full mutation,” which results in methylation and transcriptional silencing of FMR1 with consequent loss or substantial reduction in expression of the corresponding protein (FMRP) (2).