Deleted X chromosomes in patients with the fragile X syndrome.

Deleted X chromosomes in patients with the fragile X syndrome It is now well recognised that the fragile X chromo- some anomaly is associated with certain cases of X linked mental retardation.'-3The fragile site is usually seen as an isochromatid gap at the inter- face of the long arm bands Xq27 and Xq28.However, we have observed that the fragile X is also frequently manifested as a deleted X chromo- some with loss of the distal long arm band, q28 (figure).These deleted Xs may have arisen in culture as a result of mitotic non-disjunction of