JAK1 and acquired polycythemia vera: Ruxolitinib (73) is an orally administered, first-in-class Janus Kinase (JAK) 1 and 2 inhibitor that was recently approved for the treatment of patients with polycythemia vera (PV) [54], which is a rare slow-growing blood cancer characterized by increased erythrocyte production with a poorly understood etiology; IC50 values towards JAK1 and JAK2 are of 2.7, 4.5 nM, respectively [55].