In eukaryotes, a number of proteins are involved in the binding and transport of cobalamin (vitamin B12) [1]. Some of them have been sequenced and have been shown [2,3] to be evolutionary related and are listed below:

• Intrinsic factor (IF). The function of IF is to promote the absorption of cobalamin in the ileum by specific receptor-mediated endocytosis.
• Transcobalamin I (TC1) (also called R-binder protein). TC1 transports cobalamin from blood to cells.
• Transcobalamin II (TC2). TC2 transports cobalamin from blood to cells.
• Haptocorrin (cobalophilin). Haptocorrin binds to cobalamin and to cobalamin derivatives such as cobinamide. It may play a role in preventing the absorption of cobalamin analogues produced by bacteria.

These glycoproteins are polypeptides of about 400 amino acids that share many regions of similarity. As a signature pattern, we selected a highly conserved region in their central part.