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The classic BCR-ABL1-negative myeloproliferative neoplasm is an operational sub-category of MPNs that includes polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). The JAK2V617F mutation is found in ~ 95% of PV and 50-60% of ET or PMF. In most of the remaining JAK2V617Fnegative PV cases, JAK2 exon 12 mutations are present. Amongst the JAK2V617F-negative ET or PMF 5-10% of patients carry mutations in the MPL gene. Prior to 2013, there was no specific molecular marker described in the remaining 30-40% ET and PMF. In December 2013, two research groups independently reported mutations in the gene CALR found specifically in ET (67-71%) and PMF (56-88%) but not in PV. Initially CALR mutations were reported mutually exclusive with JAK2 or MPL. However, co-occurrence of CALR mutations with JAK2V617F has been reported recently in a few MPN cases. Many studies have reported important diagnostic and prognostic significance of CALR mutations in ET and PMF patients and CALR mutation screening has been proposed to be incorporated into WHO diagnostic criteria for MPN. It is suggestive in diagnostic workup of MPN that CALR mutations should not be studied in MPN patients who carry JAK2 or MPL mutations. However JAK2V617F and CALR positive patients might have a different phenotype and clinical course, distinct from the JAK2-positive or CALR-positive subgroups and identification of the true frequency of these patients may be an important factor for defining the prognosis, risk factors and outcomes for MPN patients.

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