In polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF), the major subtypes of classical myeloproliferative neoplasms (MPNs), patients possess a common somatic point mutation in JAK2 that results in the conversion of valine 617 to phenylalanine (V617F) [1–3]. Here, JAK2 is linked to acquired polycythemia vera.