TYK2 and rheumatoid arthritis: The Janus kinase (JAK) family (JAK1, 2, 3, and tyrosine kinase 2 [TYK2]) are important mediators of multiple cytokine-signaling pathways involved in normal cellular processes, as well as in the pathogenesis of rheumatoid arthritis (RA) and other immune-mediated inflammatory diseases [1–3].