The common fragile site in band q27 of the human X chromosome is not coincident with the fragile X

The common fragile site on the end of the long arm of the human X chromosome has been shown to be at a different location from the rare fragile site which produces the fragile X syndrome of intellectual handicap. The different locations can be clearly seen in chromosomes at about the 550 band level of resolution. This finding should help resolve difficulties in fragile X cytogenetics where expression of the common fragile site can lead to false positive diagnoses.