JAK1 and rheumatoid arthritis: On the other hand, it is worth noting that although all JAKi approved for RA treatment exhibit different specificity (Upadacitinib, a JAK1 inhibitor with effects on JAK2; Baricitinib, primarily a JAK1/2 inhibitor; Peficitinib, inhibitor of JAK3 over JAK1/2, and Filgotinib, primarily a JAK1 inhibitor), they all share a similar ability to re-direct macrophage differentiation and to dose-dependently induce MAFB and CD163 expression in GM-CSF-treated monocytes.