Spondyloarthritis (SpA) is an inflammatory disease that affects the gastrointestinal tract, skeleton, and eyes. HLA-B27 is a major risk gene for SpA, but the underlying mechanisms are unclear. One hypothesis is that HLA-B27 promotes SpA through misfolding-induced endoplasmic reticulum (ER) stress that in turn upregulates IL-23 expression via the transcription factor CHOP. In this study, the researchers knocked out CHOP expression in an animal model of HLA-B27-associated SpA. Despite reduced IL-23 production, gut inflammation did not improve, indicating that the gut disease did not occur as a result of ER stress-induced IL-23 production.