JAK1 and acquired polycythemia vera: The oral JAK1/2 inhibitor, ruxolitinib was the first approved targeted treatment for intermediate- or high-risk myelofibrosis (MF) on the basis of the results of the Controlled Myelofibrosis Study with Oral JAK Inhibitor Treatment-I (COMFORT-I) [7] and COMFORT-II [8] clinical trials and for patients with PV who are refractory to or intolerant of hydroxyurea on the basis of the results of the Randomized Study of Efficacy and Safety in Polycythemia Vera With JAK Inhibitor INCB018424 Versus Best Supportive Care (RESPONSE) [9] clinical trial.