Lack of functional Fragile X mental retardation protein (FMRP) is the pri-mary cause of the Fragile-mental retardation syndrome in humans. In most cases, the disease results from transcriptional silencing of fragile mental retardation gene 1, fmr1, which encodes FMRP. However, a single mis-sense mutation (I304N) in the second KH domain of FMRP gives rise to a particularly severe case of Fragile X syndrome.