Gastrointestinal melatonin has antioxidant effects, reduces the degradation of prostaglandins by prostaglandin reductase, and limits gastric lesions and hydrochloric acid secretion; it also antagonizes the actions of 5-HT, which are linked to the formation of gastric ulcers [27,28,29]. The gene discussed is HTR5A; the disease is gastric ulcer.