STAT3 and acquired polycythemia vera: Indeed, ruxolitinib, a drug currently approved for intermediate or high-risk myelofibrosis and polycythemia vera with severe leukocytosis that has failed hydroxyurea, targets the Janus kinase signal transducer and activator of transcription (JAK-STAT) signaling, thus inhibiting both the downstream pathway that mediates the neutrophil-proliferative effects of CSF [57] and the upstream pathway that induces the STAT-3-mediated IL-6 synthesis [58].