Imerslund–Gräsbeck syndrome is a rare autosomal recessive, familial form of vitamin B12 deficiency caused by malfunction of the "Cubam" receptor located in the terminal ileum.

[2][3] It has been further suggested that mutations on CUBN were restricted to exon 1-28 which encoded amnionless binding domains (EGF) and IF-Cbl binding region of cubilin, while AMN mutations primarily clustered in intron 8-11 and transmembrane domain in exon 10.

The minimal daily adult requirement is 1–3 micro gram, and the human body is able to store at any one time about 2–3 milligram, which is sufficient for at least 2 years of impeccable functioning before the source is depleted.

[citation needed] The following lists principal events that lead to absorption of vitamin B12 along the GI tract: Cubam is composed of two molecules, amnionless (AMN) and cubilin.

[9] Whereas, amnionless is an apical transmembrane protein which is expressed in both intestine and kidney, and it seems to assist the subcellular localization and endocytosis of the cubilin by binding to its amino-terminal residues.

[1] Timing is essential, as some of the side effects of vitamin B12 deficiency are reversible (such as red blood cell (RBC) indices, peripheral RBC smear findings such as hypersegmented neutrophils, or even high levels of methylmalonyl CoA), but some side effects are irreversible as they are of a neurological source (such as tabes dorsalis, and peripheral neuropathy).

[12] Studies showed that mutations in CUBN or AMN clustered particularly in the Scandinavian countries and the Eastern Mediterranean regions.