Tofacitinib, a non-selective oral Janus kinase (JAK) inhibitor, is effective in patients with UC and several other more or less selective Jak1, 2 and 3 inhibitors are being developed for the treatment of CD and UC (Sandborn et al. in N Engl J Med 376(18):1723–1736, 2017; Vermeire et al. in Lancet 389(10066):266–275, 2017; De Vries et al. in J Crohns Colitis 11(7):885–93, 2017). Here, JAK1 is linked to Crohn's colitis.