TYK2 and rheumatoid arthritis: Several known disease-specific cytokines exert their pathophysiological actions through JAK/STAT mechanisms; interleukin (IL)-6, involved in the development of rheumatoid arthritis (RA), is associated with JAK1, JAK2 and TYK23 while IL-12/IL-23, implicated in the pathogenesis of spondylarthritis (SpA) spectrum, binds to JAK2 and TYK2.2,4