Polycythemia vera (PV) and essential thrombocythemia (ET) belong to the group of BCR/ABL negative chronic myeloproliferative disorders (CMPD), which also include myelosclerosis with myeloid metaplasia, juvenile chronic myeloid leukemia, and a less-defined form of disease provisionally termed as BCR/ABL negative chronic myeloid leukemia. PV and ET are clonal hematologic neoplasias characterized by the excessive proliferation, in most instances without concomitant maturation arrest, of one predominant hematopoietic lineage, namely, the erythroid lineage in PV and the megakaryocytic lineage in ET (Dameshek, 1951; Vardiman, 1992; Hoffman and Boswell, 1995; Hoffman et al., 1995).