Constitutive activation of STAT3 was identified in a number of MPN patients from granulocytes in advance of the discovery of JAK2 V617F, whilst higher levels of STAT3 tyrosine phosphorylation have been identified in JAK2 V617F positive individuals and as a result of JAK2 V617F expression in murine models [65–67]. Here, JAK2 is linked to myeloproliferative neoplasm.