Multiple active X chromosomes in myelofibrosis with myeloid metaplasia.

A woman with myelofibrosis and myeloid metaplasia had a karyotype of 47,X,del(X)(q22),+del(X)(q22) in unstimulated peripheral blood and bone marrow aspirate cultures. The normal X chromosome was late replicating, and the two deleted X chromosomes always replicated early and synchronously. The karyotype from phytohemagglutin-stimulated peripheral blood cultures was uniformly 46,XX. Structurally abnormal X chromosomes are exceedingly rare in myeloproliferative disease. The abnormal karyotype very likely reflects monoclonal proliferation of an abnormal myeloid cell line. The X chromosome inactivation process, which acts upon embryonic somatic cells of all mammals, apparently does not react to postembryonic nondisjunction of the active X chromosome.