FMR1 and fragile X syndrome: FXS results from an expansion (>200 repeats for full mutation) and hyper-methylation of a CGG trinucleotide repeat in the FMR1 (Fragile X messenger ribonucleoprotein 1) gene, and individuals with this mutation exhibit developmental and behavioral challenges including delays in learning, speech and language delay, sensory issues, hyperactivity, and anxiety (NICHD, 2021).