CALR and acquired polycythemia vera: The four popular (also known as ‘classic') MPN are chronic myeloid leukemia, polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF).1 PV, ET and PMF are operationally referred to as BCR-ABL1-negative MPN and are characterized by recurrent JAK2, calreticulin (CALR) or myeloproliferative leukemia virus oncogene (MPL) mutations.