JAK2 and myelofibrosis: Others cannot, including failure of JAK2 V617F to enhance MPN HSC bone marrow pool size (14), resistance of MPN HSC to tyrosine kinase inhibitors (15) premature release of mutated CD34+ HSC from the bone marrow (16), myelofibrosis or the clinical presentation of PV and PMF as isolated thrombocytosis, suggesting that pathways other than constitutive JAK2 activation are involved.