Targeted treatment with the JAK1/2-inhibitor, ruxolitinib, reduces splenomegaly and constitutional symptoms significantly for some MF and PV patients, but the majority of MPN patients do not benefit from ruxolitinib treatment and discontinuation rates are high due to treatment-related anemia and thrombocytopenia [25, 26]. This evidence concerns the gene JAK1 and Splenomegaly.