FXS is the most common genetic cause of intellectual disability and autism and is caused by a CGG trinucleotide repeat expansion within the fragile X mental retardation 1 (FMR1) gene that is located on the X chromosome and codes for FMRP, a major interacting protein of CYFIP proteins (Schenck et al. 2001). The gene discussed is FMR1; the disease is autism.