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. Here, JAK2 is linked to essential thrombocythemia.