A series of relatively short (GCC)ntriplet repeats (n¼3–30) located within regulatory regions of many mammalian genes may be considered as puta-tive cis-acting transcriptional elements (GCC-elements). Fragile X-mental retardation syndrome is caused by an expansion of (GCC)ntriplet repeats within the 5¢-untranslated region of the human fragile X-mental retarda-tion 1 (FMR1) gene.