The V617F mutation of JAK2 is the key molecular event in 90% of polycythaemia vera (PV), 50% of essential thrombocythaemia (ET) and 50% of primary myelofibrosis (PMF). JAK2 exon 12 and MPLW515 mutations are less frequent. Because JAK2 V617F is specific for myeloid neoplasms, and because it can be detected in peripheral blood granulocytes, it offers a powerful tool that facilitates the diagnosis of these BCR-ABL negative myeloproLiferative disorders. These discoveries provide the rationale for a revision of the current WHO diagnostic criteria for PV, ET and PMF and could ultimately lead to the development of a specific targeted therapy.