JAK2 and primary myelofibrosis: On the other hand, mutations in the Janus kinase 2 (JAK2) gene on exon 14, mostly at codon 617 (JAK2 V617F) or other activating JAK2 mutations in exon 12 or, more rarely, mutations in the myeloproliferative leukemia (MPL) gene or in exon 9 of Calreticulin (CALR) gene are observed in Ph-negative MPNs, which may have different clinical and morphological features presenting as either polycythemia vera (PV), primary myelofibrosis (PMF), or essential thrombocythemia (ET).