ClassicBCR-ABL1–negative MPNs. The classicBCR-ABL1–negative MPNs (hereafter, MPNs) include three different disorders—essential thrombocythemia (ET), polycythemia vera (PV), and primary myelofibrosis (PMF)—and are caused by constitutive activation of the cytokine receptor/JAK2 pathway due to acquired somatic mutations in three major genes15. Here, JAK2 is linked to acquired polycythemia vera.