The majority of patients affected by classic Philadelphia chromosome-negative (or BCR-ABL1-negative) myeloproliferative neoplasms (MPNs), including polycythemia vera (PV), essential thrombocythemia (ET), and myelofibrosis (MF), harbor mutations of the genes encoding for Janus kinase 2 (JAK2), thrombopoietin receptor gene (MPL), or calreticulin (CALR) (1, 2). Here, CALR is linked to acquired polycythemia vera.