Characterisation of a new rare fragile site easily confused with the fragile X.

A new fragile site (FRAXE) in Xq28 is described. It appears to be a typical folate sensitive fragile site. The fragile site is not associated with mental retardation, it does not give abnormal results when subjected to Southern analysis with probe pfxa3 which detects the unstable DNA sequence characteristic of fragile X syndrome. In situ hybridization mapping locates the fragile site between 150 kb and 600 kb distal to FRAXA. The distinction between the two fragile sites is important clinically since cytogenetic detection of FRAXE, without molecular analysis, could result in misdiagnosis of fragile X syndrome.