[Diagnosis of fragile X syndrome]

The fragile X syndrome (FXS) is the main cause of hereditary mental retardation. Although a lower frequency has been demonstrated in latest population studies, FXS constitutes the most frequent cause of hereditary mental retardation. FXS was historically diagnosed, firstly, only in clinically affected patients, but it was not possible to detect carriers with no symptoms. Once the mechanisms that specifically induced X chromosomal fragility were beginning to be understood, it became possible to confirm clinically diagnosed patients and detect some of asymptomatic carriers. But, the discovery of FMR1 gene has allowed us to reliable know the genetic status of anyone with respect to fragile X syndrome, independently of whether one belongs to a fragile X family. In this respect a combination of molecular and cytogenetic techniques may be used to detect expansions in the repeat region of the FMR1 gene. Three classes of alleles are found, normal with 5-60 repeats, premutated with repeats between 60 and 200 copies and full mutated with expansion greater than 200 copies.