According to Dothel et al. [35], who assessed the expression of MOP and β-endorphins in the mucosal biopsies of the descending colon of 31 patients with IBS (9 with IBS-C, 10 with diarrhea-predominant IBS, and 12 with mixed IBS), the mRNA and protein expression of MOP receptor and its endogenous agonist was increased in all IBS patients as compared to the asymptomatic control group. The gene discussed is OPRM1; the disease is irritable bowel syndrome.