Fragile X syndrome is the main inheritable genetic cause of mental retardation. It is a dynamic mutation and is due to the increase in the number of CGG triplets in exon 1 of the FMR1 gene, located in Xq27.3, and to the hypermethylation of the corresponding genomic region, which impedes production of messenger RNA and, therefore, of FMRP protein. Three types of alleles can be established, according to the number of repetitions: normal, with premutation and with full mutation. Only individuals with full mutation have fragile X syndrome. Two sub-phenotypes of the syndrome, associated to premutation, have recently been reported and are seen to appear from the fourth decade of life onwards.The number of female carriers of a premutation with early ovarian failure has increased, while reports have also appeared describing a neurological picture of intentional trembling and ataxia among carriers of the premutation (FXTAS).