BCR and acquired polycythemia vera: BCR::ABL1-negative myeloproliferative neoplasms (MPNs), including polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF), are clonal hematopoietic stem cell disorders characterized by sustained myeloid proliferation and a variable risk of thrombosis, hemorrhage, and fibrotic or leukemic progression [1].