Fragile-X syndrome, the most common inherited form of mental retardation, has a very unusual mode of inheritance. The disease is caused by a multistep expansion, in successive generations, of a polymorphic CGG repeat localized in a 5' exon of FMR-1, a gene of unknown function. Two main mutation types have been categorized. Premutations are moderate expansions of the repeat and do not cause mental retardation. Full mutations are found in affected individuals and involve larger expansions of the repeat, with abnormal methylation of the neighboring CpG island. The full mutations demonstrate striking somatic instability and extinguish expression of FMR-1. Premutations are changed to full mutation only when transmitted by a female with a frequency that increases up to 100% as a function of the initial size of the premutation. Direct detection of the mutations provides an accurate test for pre- and postnatal diagnosis of the disease, and for carrier detection. A similar unstable expansion of a trinucleotide repeat occurs in myotonic dystrophy.