FMR1 and fragile X syndrome: An expansion to >200 CGG trinucleotide repeats, termed a “full mutation”, results in DNA hypermethylation of the promoter region and silencing of FMR1, leading to a deficiency in FMRP5,6 and causing Fragile X syndrome (FXS), the most common inherited cause of mental retardation and a monogenetic cause of autism1.