From a biological standpoint, a documented increase in Hb/Hct from an individual’s baseline, associated with a JAK2 mutation, should be approached as PV, even if Hb/Hct levels do not cross the WHO-defined diagnostic thresholds; such circumstances should also be considered in distinguishing PV from JAK2-mutated ET, where JAK2V617F allele burden might provide additional clue (i.e., expected to be higher in the former and often <20% in the latter). The gene discussed is JAK2; the disease is essential thrombocythemia.