Results of phase III clinical trials have demonstrated that the JAK1 and JAK2 inhibitor ruxolitinib provides significant clinical benefits for patients with advanced MF and patients with PV who have had inadequate response to or are intolerant of hydroxyurea, including but not limited to reduction of splenomegaly and symptom burden in MF and PV and hematocrit control in PV. This evidence concerns the gene JAK2 and acquired polycythemia vera.