Analysis of the human androgen receptor gene (HUMARA) allows clonality to be assessed in essential thrombocythemia (ET) and polycythemia vera (PV). We studied clonality in 44 patients with ET, 18 with PV and in 64 healthy controls. The X-chromosome inactivation pattern was analyzed by HUMARA-polymerase chain reaction on DNA from purified granulocytes, T lymphocytes and the CD3- fraction of mononuclear cells.