• Fragile X syndrome is caused by loss of function of the FMR1 gene. • 99% percent of cases of fragile X syndrome are caused by an expansion of the unstable CGG repeat sequence in the 5′ untranslated region (UTR) of the FMR1 gene. • Features include strabismus, seizures, musculoskeletal abnormalities (pes planus, pectus excavatum, scoliosis), cardiovascular phenotypes (mitral valve anomalies, aortic root dilatation), and behavioral disturbances. This evidence concerns the gene FMR1 and fragile X syndrome.