JAK2 and primary myelofibrosis: The myeloproliferative neoplasms (MPN), polycythemia vera (PV), essential thrombocytosis (ET) and primary myelofibrosis (PMF) are clonal hematopoietic stem cell (HSC) disorders that share gain of function mutations which directly or indirectly constitutively activate JAK2 [1–4], the cognate tyrosine kinase of the erythropoietin (EPO) and thrombopoietin (THPO) receptors [5], and also utilized by the granulocyte colony-stimulating factor receptor [6].