Key Points• The tumor suppressor PP2A is repressed in Jak2 V617F -driven myleoproliferative neoplasms by a Jak2/PI3K/ PKC/SET signaling pathway.• PP2A-activating (eg, FTY720, OSU-2S) but not sphingosine-1-phosphate agonistic (eg, FTY720-P) drugs selectively kill Jak2 V617F1 cells.FTY720 (Fingolimod, Gilenya) is a sphingosine analog used as an immunosuppressant in multiple sclerosis patients. FTY720 is also a potent protein phosphatase 2A (PP2A)-activating drug (PAD). PP2A is a tumor suppressor found inactivated in different types of cancer. We show here that PP2A is inactive in polycythemia vera (PV) and other myeloproliferative neoplasms characterized by the expression of the transforming Jak2 V617F oncogene. PP2A inactivation occurs in a Jak2 V617F dose/kinase-dependent manner through the PI-3Kg-PKC-induced phosphorylation of the PP2A inhibitor SET. Genetic or PADmediated PP2A reactivation induces Jak2 V617F inactivation/downregulation and impairs clonogenic potential of Jak2 V617F cell lines and PV but not normal CD34 1 progenitors.Likewise, FTY720 decreases leukemic allelic burden, reduces splenomegaly, and significantly increases survival of Jak2 V617F leukemic mice without adverse effects. Mechanistically, we show that in Jak2 V617F cells, FTY720 antileukemic activity requires neither FTY720 phosphorylation (FTY720-P) nor SET dimerization or ceramide induction but depends on interaction with SET K209. Moreover, we show that Jak2 V617F also utilizes an alternative sphingosine kinase-1-mediated pathway to inhibit PP2A and that FTY720-P, acting as a sphingosine-1-phosphatereceptor-1 agonist, elicits signals leading to the Jak2-PI-3Kg-PKC-SET-mediated PP2A inhibition. Thus, PADs (eg, FTY720) represent suitable therapeutic alternatives for Jak2