JAK2 and acquired polycythemia vera: The genetic basis of MPNs has been extensively studied: up to 95% of patients with PV and over 50% of those with ET or MF have a mutation in the Janus kinase 2 (JAK2) gene, exon 14, in which valine is replaced with phenylalanine in position 617 (JAK2V617F), resulting in activation of biochemical pathways implicated in erythropoietin receptor signaling13–19.