Megaloblastic anemia is a hematological disorder in which the bone marrow produces abnormally large nucleated cells, including immature erythrocytes, superlobulated polymorphic leukocytes and large platelets. At the cellular level, megaloblastic anemia 1 (MGA1), originally named Imerslund-Grasbeck syndrome (IGS), is caused by intestinal malabsorption of vitamin B12 (cobalamin) due to a defect in the vitamin B12/gastric intrinsic factor (GIF) complex receptor. Clinically, IGS is characterized by relatively early childhood onset (before age 5 years), failure to thrive, infections, low level of vitamin B12 in serum, mild and innocuous proteinuria, and variable neurological symptoms such as peripheral neuropathy, cognitive problems, and dementia.