Polycythemia vera, the most common form of primary polycythemia, is caused by somatic mutation in a single hematopoietic stem cell leading to clonal hematopoiesis. PV is a myeloproliferative disorder characterized predominantly by erythroid hyperplasia, but also by myeloid leukocytosis, thrombocytosis, and splenomegaly. Familial cases of PV are very rare and usually manifest in elderly patients. PV is distinct from the familial erythrocytoses, which are caused by inherited mutations resulting in hypersensitivity of erythroid progenitors to hormonal influences or increased levels of circulating hormones, namely erythropoietin [from OMIM].