Kovacsovics-Bankowski et al. analyzed patients with polycythemia vera (PV) and essential thrombocythemia (ET) and found increased numbers of circulating Tregs and an enrichment in highly suppressive subsets (defined as CD39+/HLA-DR+) in patients treated with PegIFN-α with respect to those treated with hydroxyurea [67]. Here, ENTPD1 is linked to acquired polycythemia vera.