The Janus kinase 2 (JAK2) V617F mutation is an activating JAK2 tyrosine kinase mutation detected in the majority of MPN patients with polycythemia vera (PV), and in half of those with essential thrombocytemia (ET) and primary myelofibrosis (MF) [1–4]. Here, JAK2 is linked to primary myelofibrosis.