Fragile X syndrome (FXS) is a one of the most common autism spectrum disorders (ASDs) and results from mutations in the X-linked FMR1 gene: the 5’ untranslated region of FMR1 contains a CGG-triplet repeat, whose abnormal expansion (> 200 repeats) can cause epigenetic silencing of FMR1 and thus a loss of FMR1-encoded fragile X messenger ribonucleoprotein 1 (FMRP). This evidence concerns the gene FMR1 and autism spectrum disorder.