Fragile X syndrome is the most common cause of inherited mental retardation (about 1/4,000 affected males). The syndrome is caused by the abnormal expansion of a small piece of gene, FMR1, consisting of a sequence repeated several times and located on the X chromosome. Because it is linked to the X chromosome, males are usually more affected than females. The disease is only present if the expansion exceeds a certain number of repetitions (complete mutation). Between the normal number of repetitions and the complete mutation are the “intermediate” and “premutated” zones. It is only the latter, when unstable – i.e. lengthening with the passing of generations – that causes the syndrome in subsequent generations. Instability is practically only found in the transition from mother to son/daughter and not from father to son/daughter. In the Caucasian population, premutation carriers are estimated to be around 1/150-250. The test to determine if premutation is present is very important in order to establish if there is a risk of conceiving a child with the syndrome, and in such case propose a prenatal or preimplantation analysis. The result is also very important in determining whether there are other women at risk of transmitting the disease within the family.