A woman with an abnormal karyotype, (46,X,der(X) (pter----q27::q27----q21), was analyzed using DNA probes in the region Xq27----qter. The results indicate that she is trisomic for the Factor IX locus, disomic for the locus DXS105 (cX55.7) and monosomic for the loci DXS98 (4D8), DXS52 (St14) and Factor VIII. This confirms the absence of the region Xq28 in the abnormal chromosome. Furthermore, the presence of only one copy of 4D8 and two copies of cX55.7 places the DXS98 locus distal to Factor IX and closer to the fragile X locus than DXS105.