JAK inhibitors available for the treatment of patients with RA include tofacitinib (a pan-JAK inhibitor approved in the USA, Europe, and Asia [5–8]), baricitinib (a JAK1 and JAK2 selective inhibitor approved in the USA, Europe and Asia [9–12]), and more recently peficitinib (ASP015K; approved in Japan). This evidence concerns the gene JAK2 and rheumatoid arthritis.