The most common causes of vitamin B12 deficiency include pernicious anemia, gastritis, impaired absorption secondary to surgery or inflammatory bowel disease, pancreatic insufficiency, vegan diets, drugs that block absorption such as inhibitors of proton pump inhibitor, biguanide neomycin, anesthesia with nitric oxide or genetic disorders such as deficiency of transcobalamin II. The gene discussed is TCN2; the disease is gastritis.