JAK2 and Thrombocytosis: The V617F JAK2 mutation encoding a cytoplasmic tyrosine kinase, has been described in 5% of MDS associated with megakaryocytic proliferation, and 50% of MDS/MPN overlapping refractory anemia with ring sideroblasts and thrombocytosis (MDS/MPN-RS-T) (37, 63) The overall prognostic significance of JAK2 is unclear in MDS, but targeting similar to that in MPN is not irrational.