Sex chromosome marker: clinical significance and DNA characterization.

Two patients are described with a virtually identical marker sex chromosome that was so small as to defy classification by conventional cytogenetic studies. DNA hybridization with Y probes allows classification into Y or non-Y, and in situ hybridization with X centromere specific sequences, into X or non-X. One patient was proven to have a Y fragment, and the second, an X fragment. DNA characterization is important since prognosis and clinical management depends on proper identification of the small marker sex chromosome.