In 2005 major breakthrough in our understanding of the molecular pathophysiology was achieved with the identification of the JAK2 V617F mutation which is present in almost all patients with PV (98%) and about half of patients with ET and PMF. This somatic gain-of-function point mutation in the JAK2 tyrosine kinase leads to constitutive activation of the kinase. By this mechanism a clonal non-growth factor dependent myeloproliferation is established.