For example, IBS patients may experience excessive proliferation of pathogenic bacteria, a decrease in the proportion of probiotics such as lactic acid bacteria and bifidobacteria, disruption of gut microbiota leading to hyperfunction of the HPA axis, and disorder in the regulation of brain–gut peptides such as 5-HT, CRH, and substance P (SP). The gene discussed is TFF2; the disease is irritable bowel syndrome.