FMR1 and fragile X syndrome: Silencing of FMR1 is nearly always caused by hypermethylation of a cytosine guanine guanine (CGG) trinucleotide repeat expansion (≥200 repeats is termed the “full mutation” and causes FXS) in the 5′ untranslated region (UTR) of the FMR1 gene [6, 7].