Abstract: Polycythemia vera (PV) is a myeloproliferative neoplasm (MPN) characterized by overproduction of red blood cells. The pathogenesis of PV is not completely understood. However, studies report that it may be associated with the gain-of-function somatic mutation of JAK2 and that the JAK2 mutation provides a molecular diagnostic standard for PV. JAK2 mutation and allele mutation burden are useful for predicting clinical features and courses. The discovery of JAK2 mutation has promoted the development of molecular-targeted therapy, such as the JAK2 inhibitor, ruxolitinib, a drug with superior therapeutic effect and safety that is used in clinical practice. The JAK2 allele mutation burden is closely associated with leukocytosis and progression to myelofibrosis (MF). A high JAK2 allele mutation burden may be a risk factor for poor prognosis. This article briefly reviews the clinical significance of the JAK2 mutation in patients with PV.