PV, the commonest MPN, is characterized with erythrocytosis in consequence of JAK2 exon 14 or 12 somatic mutations (Baxter et al., 2005; James et al., 2005; Kralovics et al., 2005; Levine et al., 2005; Scott et al., 2007). Here, JAK2 is linked to acquired polycythemia vera.