JAK2 and acquired polycythemia vera: Ruxolitinib, a JAK1 and JAK2 inhibitor, provides clinical benefit, including hematocrit and WBC count control and reductions in splenomegaly and symptom burden, and was approved by the US Food and Drug Administration in 2014 for PV in adults who have an inadequate response to or are intolerant of HU [22–29].