Transcobalamin (TCN, TC) is a vitamin B12-binding protein secreted by endothelial cells into plasma that facilitates the endocytosis of cobalamin (Cbl, vitamin B12) into hepatocytes or cells requiring Cbl. Two TCN genes, TCN1 (aka haptocorrin) and TCN2, code for functional proteins (TCI and TCII respectively) that can bind Cbl (Johnston et al. 1989, Quadros et al. 1986, Wuerges et al. 2006). TCII can be bound to between 10-30% of the total circulating Cbl, the remaining Cbl bound to TCI and not available for uptake by cells outside of the liver. TCII transports Cbl used by tissues. The role of TCI carrying between 70-90% of the Cbl serum fraction is unknown. Free Cbl can be taken up by passive diffusion but only at concentrations that are never achieved in the body.