FMR1 and fragile X syndrome: Most FXS cases are the result of an unstable trinucleotide repeat (CGG) expansion in the 5'-untranslated region of FMR1. When of sufficient size, these expansions cause promoter methylation and gene silencing, resulting in the absence of, or reduced levels of FMR1 mRNA and FMRP (the protein product of FMR1).