JAK2 and primary myelofibrosis: The JAK2V617F mutation, which occurs in most patients with polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF), is considered integral to the pathogenesis of myeloproliferative neoplasms (MPNs). 1-3  There is now a growing interest in the JAK2 V617F allele burden (% JAK2 V617F) and its potential influence on disease phenotype.