The myeloproliferative disorders, comprising polycythemia vera, essential thrombocytosis, and myelofibrosis, are clonal hematopoietic cancers that have an indolent course. The clinical manifestations of these entities overlap, as do their genetic drivers. The myeloproliferative neoplasms — polycythemia vera, essential thrombocytosis, and primary myelofibrosis — are unique hematopoietic stem-cell disorders that share mutations that constitutively activate the physiologic signal-transduction pathways responsible for hematopoiesis (Table 1). Consequently, these disorders engage in phenotypic mimicry among themselves, as well as with myeloid neoplasms and even benign hematopoietic disorders. In contrast to the myeloid neoplasms, the myeloproliferative neoplasms have a natural history, with supportive care alone, that is usually measured in decades rather than years. 1 However, a facade of benign myeloproliferation masks a clone of transformed hematopoietic stem cells capable of expansion and transformation to an aggressive . . .