JAK2 and rheumatoid arthritis: A series of small molecule inhibitors of JAK isoforms have been approved for treating RA since 2009 (Choy, 2019), including tofacitinib (the first JAK1/JAK3 inhibitor), baricitinib (JAK1/JAK2 inhibitor), upadacitinib and filgotinib (JAK1 inhibitor) and peficitinib (pan-JAK inhibitor) (Norman, 2014; Kaneko, 2020).