Evolution of essential thrombocytosis and primary myelofibrosis into polycythemia vera. (A) Erythrocytosis developing in a 60-year-old man with essential thrombocytosis 6 years after diagnosis. The increase in the JAK2 V617F neutrophil allelic burden with time is also shown. The hemoglobin (Hgb) level was reduced by phlebotomy. (B) Erythrocytosis developing in a 70-year-old woman with classic PMF of 17 years' duration, while taking hydroxyurea to control splenic enlargement (bracketed line). The hemoglobin (Hgb) level was reduced by phlebotomy.