Tofacitinib, a JAK3 inhibitor approved for clinical use in rheumatoid arthritis [19], [20], is particularly interesting as it possesses considerable specificity across the kinome: In a 317 kinase panel, tofacitinib only inhibits 3 kinases with an IC50 <500 nM: JAK3, JAK2 and PRK1 [21]. This evidence concerns the gene JAK3 and rheumatoid arthritis.