The classic BCR-ABL1–negative 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 genes [1]. The gene discussed is JAK2; the disease is acquired polycythemia vera.