While the BCR-ABL fusion protein is the transforming agent in CML, driver mutations in JAK2, CALR, and MPL genes are variably present and are mostly mutually exclusive in Ph−MPNs, which include essential thrombocythemia (ET), polycythemia vera (PV), and primary myelofibrosis (MF) [41]. The gene discussed is CALR; the disease is acquired polycythemia vera.