The JAK2 V617F point mutation, though rare, is implicated in the pathogenesis of hypereosinophilic states associated with MPNs such as polycythemia vera (PV) and essential thrombocythemia (ET), and was identified in 4% of cases in the German Registry study of 426 patients with hypereosinophilia of unknown significance, alongside 3% with a mutation in the KIT gene encoding the tyrosine kinase receptor (D816V) [7,24]. Here, JAK2 is linked to essential thrombocythemia.