Additionally, the reactivation of the FMR1 gene through dCas9–Tet1-induced demethylation of cytosines in CGG repeats (Liu et al., 2018) or dCas9–VP192-induced transcriptional activation of the FMR1 promoter (Haenfler et al., 2018) restored the spontaneous hyperactivity in neurons derived from human induced pluripotent stem cells (hiPSCs) of patients affected by Fragile X syndrome (FXS). This evidence concerns the gene TET1 and fragile X syndrome.