Fragile site Xq27.3 in a family without mental retardation.

Routine parental blood analysis for a couple undergoing prenatal diagnosis because of maternal age, revealed a 47,XXX karyotype in the mother and expression of the fragile site Xq27.3 in the father. Additional family studies show the fragile site in the father's sister and her two sons. There is no history of intellectual handicap in this family, nor of any physical manifestations of the Fra(X) mental retardation syndrome.