FMR1 and fragile X syndrome: Previous studies in FXS hESC show that some full mutation hESC remain unmethylated following derivation and exhibit gradual loss of FMR1 mRNA during directed neuronal differentiation (Eiges et al., 2007; Telias et al., 2013; Colak et al., 2014), similar to the silencing observed in human FXS fetuses (Malter et al., 1997).