TO THE EDITOR:

Myeloproliferative neoplasms (MPNs) are clonal hematopoietic stem cell (HSC) diseases characterized by increased proliferation of erythroid, megakaryocytic, and/or myeloid lineages. The JAK2-V617F mutation can be found in >95% of polycythemia vera (PV) patients, and also in approximately one-half of patients with essential thrombocythemia (ET) or primary myelofibrosis (PMF).2-5  Somatic mutations in exon 12 of JAK2 are found in 3% to 5% of PV patients. Quantification of the JAK2-mutant allele burden, also called variant allele frequency (VAF), in DNA from peripheral blood granulocytes is used to monitor the size of the mutant clone. ET patients have lower JAK2 VAF than PMF or PV patients. Interestingly, some MPN patients display very low VAF, which calls into question why they develop MPNs if the clone is apparently unable to expand. We therefore studied MPN patients...

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