The mechanism underlying FMR1 inactivation is of particular interest since FMR1 reactivation can serve as a therapeutic strategy for FXS.186,187 Despite intensive research, the exact mechanism of the repeat expansion-induced gene silence remains unresolved.188 One important factor is the DNA cytosine hypermethylation of the FMR1 promoter and the repeat region. The gene discussed is FMR1; the disease is fragile X syndrome.