The molecular pathogenesis of chronic myeloproliferative disorders (MPDs) is poorly understood. The hematopoietic progenitor cells of patients with polycythemia vera (PV) or essential thrombocythemia (ET) are characterized by hypersensitivity to hematopoietic growth factors and formation of endogenous erythroid colonies. Recently, 4 groups reported almost simultaneously Janus kinase 2 (JAK2) V617F mutation in more than 80% of PV patients, 30% of patients with ET and in about 50% of patients with idiopathic myelofibrosis. The identification of the JAK2 mutation represents a major advance in the understanding of the molecular pathogenesis of MPDs that will likely permit a new classification and the development of novel therapeutic strategies for these diseases.