JAK2 and rheumatoid arthritis: However, each JAK inhibitor currently approved for RA has different activities against JAK1−3 as follows: tofacitinib is preferentially a JAK 1 and 3 inhibitor, baricitinib is primarily a JAK 1 and 2 inhibitor, while upadacitinib selectively inhibits JAK1 which could potentially reduce JAK2 and JAK3-related side effects (Winthrop, 2017).