JAK2 and essential thrombocythemia: The prePMF is classified by WHO as major criteria: megakaryocytic proliferation and atypia, without reticulin fibrosis > grade 1, accompanied by increased age-adjusted BMA cellularity together with granulocytic proliferation and decreased erythropoiesis; not meeting the WHO criteria for BCR-ABL1 CML, PV, ET, MDS or other myeloid neoplasms plus presence of JAK2 V617F, CALR or MPL mutations [8].