A somatic point mutation (V617F) in the JAK2 tyrosine kinase was found in a majority of patients with polycythemia vera (PV), essential thrombocythemia, and primary myelofibrosis. However, contribution of the JAK2V617F mutation in these 3 clinically distinct myeloproliferative neoplasms (MPNs) remained unclear. To investigate the role of JAK2V617F in the pathogenesis of these MPNs, we generated an inducible Jak2V617F knock-in mouse, in which the expression of Jak2V617F is under control of the endogenous Jak2 promoter. Expression of heterozygous mouse Jak2V617F evoked all major features of human polycythemia vera (PV), which included marked increase in hemoglobin and hematocrit, increased red blood cells, leukocytosis, thrombocytosis, splenomegaly, reduced serum erythropoietin (Epo) levels and Epo-independent erythroid colonies. Homozygous Jak2V617F expression also resulted in a PV-like disease associated with significantly greater reticulocytosis, leukocytosis, neutrophilia and thrombocytosis, marked expansion of erythroid progenitors and Epo-independent erythroid colonies, larger spleen size, and accelerated bone marrow fibrosis compared with heterozygous Jak2V617F expression. Biochemical analyses revealed Jak2V617F gene dosage-dependent activation of Stat5, Akt, and Erk signaling pathways. Our conditional Jak2V617F knock-in mice provide an excellent model that can be used to further understand the molecular pathogenesis of MPNs and to identify additional genetic events that cooperate with Jak2V617F in different MPNs.