FMR1 and fragile X syndrome: Most cases of FXS are caused by CGG trinucleotide repeat expansion (more than 200 repeats) within the promoter of the fragile X messenger ribonucleoprotein 1 gene (FMR1) gene, leading to promoter hypermethylation, transcriptional silencing, and decreased expression of the fragile X messenger ribonucleoprotein protein (FMRP) (6).