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Defects in AMN cause recessive hereditary megaloblastic anemia 1 (RH-MGA1 aka MGA1 Norwegian type or Imerslund-Grasbeck syndrome, I-GS; MIM:261100). The Norwegian cases described by Imerslund were due to defects in AMN (Imerslund 1960). The resultant malabsorption of Cbl (vitamin B12) leads to impaired B12-dependent folate metabolism and ultimately impaired thymine synthesis and DNA replication.