In contrast to chronic myeloid leukaemia, the molecular mechanisms leading to Ph- MPN progression have remained unclear until the recent finding of the V617F JAK2 mutation in most of Polycythemia Vera (PV) and half of Essential Thrombocytosis (ET) and Primary Myelofibrosis (PMF) cases ([3], see for review [4]). Here, JAK2 is linked to acquired polycythemia vera.