In-depth analysis of the MHC region including classical HLA alleles showed that the strongest signal for disease location was a colonic association with HLA-DRB1*01:03 (p=1·47 × 10−23; ileal vs colonic odds ratio [OR] 0·32, 95% CI 0·29–0·41; ileocolonic vs colonic OR 0·47, 0·39–0·57), which is also the strongest shared risk allele for Crohn's disease and ulcerative colitis,35 followed by HLA-DRB1*07:01 (figure 2; appendix A). This evidence concerns the gene HLA-DRB1 and ulcerative colitis.