Necessary for efficient absorption of vitamin B12. May direct the production of trunk mesoderm during development by modulating a bone morphogenetic protein (BMP) signaling pathway in the underlying visceral endoderm. Defects in AMN are a cause of recessive hereditary megaloblastic anemia 1 (RH-MGA1); also known as MGA1 Norwegian type or Imerslund-Grasbeck syndrome (I-GS). RH-MGA1 is due to selective malabsorption of vitamin B12. Defects in vitamin B12 absorption lead to impaired function of thymidine synthase. As a consequence DNA synthesis is interrupted. Rapidly dividing cells involved in erythropoiesis are particularly affected. 1 isoforms of the human protein are produced by alternative promoter. Note: This description may include information from UniProtKB.