Fragile sites have also been implicated in a variety of syndromes (for a review, see ). For example, breakage at or near the FRA11b locus has been implicated in Jacobsen syndrome, which is characterized by loss of part of the long arm of chromosome 11 accompanied by mild mental retardation. The FRAXE site is associated in the development of a form of mental retardation without any distinctive phenotypic features. Seckel syndrome, a genetic disease characterized by low levels of ATR, results in increased instability of chromosomes at fragile sites.