FMR1 and fragile X syndrome: Xie et al. (2016) utilized the CRISPR genome editing technology to excise the expanded CGG repeat from the full mutation allele in FXS cells resulting in an FMR1 allele without CGG repeats. The excision of the expanded CGG-repeat from the fragile X chromosome resulted in FMR1 reactivation thereby restoring FMRP production. Liu et al. (2018) applied recently developed DNA methylation editing tools and demethylated the CGG expansion by dCas9-Tet1/single guide RNA (sgRNA).