Models to explain the relationship of essential thrombocythemia to polycythemia vera. (A) At the population level, different pathogenetic lesions are likely to be associated with overlapping distributions of hemoglobin level. In keeping with this model, a proportion of patients harboring a JAK2 V617F-homozygous clone may not manifest erythrocytosis; conversely, erythrocytosis may be seen in the absence of a V617F-homozygous clone. (B) When considering an individual with JAK2-mutant disease, the presence or absence of erythrocytosis likely reflects interplay between JAK2 mutation type, gene dosage and additional factors such as patient gender, iron homeostasis and inherited or acquired genetic modifiers.
 Het indicates heterozygous; hom: homozygous.