Joining efforts of the German IBS research network and the European network GENIEUR (The Genes in Irritable Bowel Syndrome Research Network Europe; www.GENIEUR.eu) enabled us to perform a multi-center study to validate previous association findings of functional SNPs of the serotonin type 3 receptor subunit genes HTR3A c.-42C > T (rs1062613), HTR3B p.Tyr129Ser (rs1176744), HTR3C p.N163K (rs6766410), and HTR3E c.*76G > A (rs56109847 = rs62625044) (illustrated in Fig. 1) and provide further evidence for the relevance of serotonin type 3 receptors to the aetiology of IBS. This evidence concerns the gene HTR3B and irritable bowel syndrome.