FMR1 and fragile X syndrome: In fact, of the FXS hESC lines examined so far, the majority present some levels of methylation (Avitzour et al., 2014), although no line was observed with completely methylated (100%) or entirely transcriptionally inactive FMR1. Nevertheless, the finding that most of these cell lines are, at least in part, already methylated raises doubts as to the actual timing of epigenetic gene silencing in FXS.