Polycythemia vera (PV) is a Philadelphia chromosome-negative myeloproliferative neoplasm (MPN) caused by a somatic gain-of-function mutation of the Janus kinase2 (JAK2) gene in hematopoietic stem and peripheral blood cells, leading to erythrocytosis which is often accompanied by leukocytosis and thrombocytosis. The gene discussed is JAK2; the disease is myeloproliferative disorder.