CALR and acquired polycythemia vera: Polycythemia vera (PV), essential thrombocythemia (ET), and myelofibrosis (MF) including pre-fibrotic/early primary myelofibrosis (PMF) are classical Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs) characterized by the uncontrolled clonal proliferation of hematopoietic stem or progenitor cells due to driver mutations of genes including JAK2, CALR, or MPL (1, 2).