JAK2 and myeloproliferative neoplasm: 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.