Conversely, Janus kinase 2 (JAK2) mutations, especially JAK2 V617F, are frequently encountered in Ph negative MPN, particularly in polycythemia vera (>95%) [1–3], primary myelofibrosis (50%) [3], and in essential thrombocythemia (40 to 50%) [1]. Here, JAK2 is linked to primary myelofibrosis.