It is noteworthy that persistent accumulation of DNA SBs and elevated p53-dependent apoptosis have also been found to be an important pathogenic feature in Fragile X syndrome (one of the most common form of inherited mental retardation), which is caused by the transcriptional silencing of fragile X mental retardation protein (FMRP;[51]). This evidence concerns the gene FMR1 and fragile X syndrome.