A post-gastroenteritis syndrome would be self-limiting (“watch and wait”); small intestinal bacterial overgrowth could be sought with an H2-breath test and analysis of duodenal aspirate, and treated with oral antibiotics; an autoimmune enteropathy could be diagnosed by an experienced gastrointestinal pathologist’s review of the biopsies and enterocyte antibodies; and in case of Zollinger–Ellison syndrome, serum gastrin would be increased, usually together with a tumor in the pancreas or duodenal wall. The gene discussed is GAST; the disease is neoplasm.