JAK2 and acquired polycythemia vera: Ruxolitinib is a potent JAK2 inhibitor that is widely used to treat polycythemia vera and myelofibrosis.6,7 Studies have evaluated this drug in cell lines with a variety of JAK2 fusion variants, including PCM1-JAK2, and found it to be active at nanomolar concentrations.11,12,33,38 Ruxolitinib has produced significant clinical responses in this type of MPN, including hematologic remissions.