JAK2 and primary myelofibrosis: Myeloproliferative neoplasms (MPNs), polycythemia vera (PV), essential thrombocytosis (ET), and primary myelofibrosis (PMF) are an unusual group of myeloid neoplasms since they share three gain-of-function somatic driver mutations, JAK2, CALR and MPL, and, as a consequence, there is an inappropriate increase in the production of morphologically and functionally normal red cells, leukocytes, and platelets either together or alone, depending on the particular MPN.