The majority of Fragile X syndrome (FXS; OMIM 300624) cases are caused by a full trinucleotide (CGG) repeat expansion of more than 200 repeats in the 5′ untranslated region of FMR1 that results in hypermethylation of FMR1 and loss of expression of the RBP FMRP [73,74,75,76]. This evidence concerns the gene FMR1 and fragile X syndrome.