Vitamin B12 (Cobalamin) in serum is bound to two proteins: Transcobalamin (TC) and Haptocorrin (HC). The transcobalamin-vitamin B12 complex is called Holotranscobalamin (HoloTC). HoloTC contains the biologically available cobalamin as only HoloTC promotes the uptake of cobalamin by all cells via specific receptors. In comparison, approximately 80% of the cobalamin carried by HC is considered metabolically inert because no cellular receptors exist, with the exception of receptors found in the liver. The shorter circulating half-life of HoloTC compared to Holohaptocorrin (HoloHC) makes a decrease of HoloTC one of the earliest markers of cobalamin deficiency.