FMR1 and fragile X syndrome: FXS is caused by a full expansion of the CGG repeats (≥200 repeats) in the 5′ region of the fragile X messenger ribonucleoprotein 1 (FMR1) gene, resulting in the loss of function of the gene and the consequent deficiency or absence of the FMR1 protein (FMRP) [1].