IL9 and acquired polycythemia vera: Compared to PV, ET was characterized by high levels of four T-cell-derived interleukins (IL-4, IL-9, IL-17, IL-26) and four cytokines produced by monocytes–macrophages: monokine induced by IFN-γ (MIG), TGF-β1, TGF-β2 and TGF-β3 (Figure 1b–h, Table 2).