Somatic JAK2 point mutations have been shown previously to be bona fide cancer-initiating lesions in myeloproliferative neoplasms (MPNs) (Baxter et al. 2005; Levine et al. 2005; Lacout et al. 2006; Wernig et al. 2006), where ∼95% of polycythemia vera (PV) and 50% of essential thrombocythemia (ET)/primary myelofibrosis (PMF) cases harbor the recurrent activating JAK2V617F alteration (Baxter et al. 2005; James et al. 2005; Kralovics et al. 2005; Levine et al. 2005). Here, JAK2 is linked to acquired polycythemia vera.