Crohn’s disease and ulcerative colitis are chronic, immune-mediated inflammatory diseases of the gastrointestinal tract, affecting up to 0.4% of the population in Western countries.1 They are considered complex multifactorial polygenic diseases. While dramatic progress has been made in deciphering the genetic architecture of Crohn’s disease, ulcerative colitis was somewhat left behind, with virtually no relevant studies published until very recently. Although both disorders are considered to belong to the same spectrum of pathologies, the first gene clearly associated with Crohn’s disease (CARD-15)2 3 did not seem to predispose to ulcerative colitis in any cohort in which it was tested. This finding highlighted some pathogenic differences between the two diseases, and reminded geneticists working on inflammatory bowel diseases (IBDs) of the more modest contribution of genetics to the predisposition to ulcerative colitis, reflected in a lower ratio of disease concordance between monozygotic and dizygotic twins.4 The lower heritability of ulcerative colitis also accounts for the fact that most of the linkage studies performed in the 1990s did not include sufficient ulcerative colitis-affected sib pairs to disclose ulcerative colitis-related loci. With few exceptions,5 6 7 reports of associations with candidate genes have not been confirmed. Recently, major advances in cataloguing common genetic variants in humans combined with the development of high-throughput single nucleotide polymorphism (SNP) genotyping capacity, have made large-scale, genome-wide association studies (GWASs) with hundreds of thousands of common SNPs feasible. These have dramatically increased the list of loci shown to be associated with Crohn’s disease,8 9 10 11 12 13 and now also with ulcerative colitis14 15 16 17 (table 1).