Tofacitinib, with specificity for JAK1 and JAK3, reduces IL-23 triggered IL-17A secretion from T cells of rheumatoid arthritis (RA) and psoriatric arthritis (PSA) patients in vitro13,14, and was the first JAK inhibitor to be licensed for treatment of RA in the United States15,16. This evidence concerns the gene JAK1 and rheumatoid arthritis.