A 66-year-old man who presented with progressive and marked thrombocytosis but normal hemoglobin was diagnosed to have essential thrombocythemia upon the demonstration of JAK2 V617F mutation. Bone marrow examination, however, showed the presence of monolobulated megakaryocytes and conventional cytogenetic analysis revealed an isolated interstitial deletion of the long arm of chromosome 5, characteristic of 5q- syndrome. A literature review indicated that isolated deletion of 5q is uncommon in essential thrombocythemia but that, when this isolated deletion is present, the disease often shows mixed features of both essential thrombocythemia and 5q- syndrome.
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