These hematologic malignancies, typified by clonal proliferation of myeloid lineage cells, include polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF or MF), which are variably associated with mutations in the Janus Kinase 2 (Jak2) gene [2]. The gene discussed is JAK2; the disease is essential thrombocythemia.