PPI: proton pump inhibitors; MMA: methylmalonic acid; holoTC: holotranscobalamin; Hcy: homocysteine; H2RAs: histamine receptor antagonists; ZES: Zollinger-Ellison syndrome; MCV: mean corpuscular volume; GERD: gastro-esophageal reflux disease; H. pylori: Helicobacter pylori. The gene discussed is ATP12A; the disease is Zollinger-Ellison syndrome.