While phase II-III trials clearly demonstrated efficacy of tofacitinib, an oral small molecule pan-JAK inhibitor, in patients with UC, in CD tofacitinib did not differ from placebo, raising questions about the design of those trials but also about tofacitinb elicited biological mechanisms that can be beneficial for only UC. Subsequently, second generation JAK-1 selective inhibitors such as filgotinib were found efficacious in CD, underscoring lack of understanding how JAK inhibitors work in IBD and which patients are more likely to benefit from their use.