Concerning the role of X‐inactivation and DNA methylation in fragile X syndrome

Elucidation of the role of DNA methylation in X chromosome inactivation along with recent studies of the fragile X mutation suggests that DNA methylation is likely to be a late event in the pathogenesis of the fragile X syndrome. Thus far, the evidence does not support suggestions that an impediment to X reactivation and failure to demethylate the inactive X in oocytes is responsible for silencing the fragile X. The role of DNA methylation is probably secondary to amplification of the CGG repeat to a critical size whether on active or inactive X. Further studies are needed to determine if late replication of the inactive X predisposes the locus on that chromosome to more extensive amplification.