CD34 and acquired polycythemia vera: The BCR-ABL1-negative myeloproliferative neoplasms primary myelofibrosis (PMF) and advanced forms of essential thrombocythemia and polycythemia vera (i.e., post ET/PV myelofibrosis) are chronic hematological malignancies characterized by splenomegaly, leukoerythroblastosis, extramedullary hematopoiesis and constitutive mobilization of CD34-positive progenitor cells.