The fragile X syndrome belongs to the most common genetic diseases and has a prevalence of one in every 2000 children. The syndrome is named after the fragile site in q27.3 on the X chromosome. The molecular cloning of the DNA containing the fragile site has resulted in the identification of a heritable unstable DNA sequence revealing a new mechanism of mutation in human genetic disorders. This DNA sequence significantly facilitates the diagnosis and provides a rapid method for carrier detection and prenatal diagnosis. The unstable element is located within a candidate gene, FMR1. The FMR1 protein is not made in fragile X patients and nothing is known about its function. We will have to await studies on this protein to be able to understand the variable phenotype of this disease.