JAK1 and acquired polycythemia vera: Tofacitinib primarily inhibits JAK1 and JAK3 and is an FDA-approved treatment for RA, with clinical trials ongoing for IBD [201]; ruxolitinib is selective for JAK1 and JAK2, and is approved for myelofibrosis and polycythaemia vera [202]; and parcritinib is a JAK2 inhibitor which is currently in phase II trials for myelofibrosis [204].