CALR and essential thrombocythemia: Essential thrombocythemia (ET), polycythemia vera (PV), and primary myelofibrosis (PMF) are a group of Philadelphia (Ph)-negative myeloproliferative neoplasms (MPN) characterized by overlapping clinical and laboratory features, as well as common phenotypic driver mutations in the JAK2, CALR and MPL tyrosine kinase genes.