Vitamin B12 deficiency occurs from inborn metabolic errors, alterations of B12-binding proteins including haptocorrin (HC) found in saliva, intrinsic factor (IF) produced by parietal cells in the stomach (pernicious anemia is associated with anti-parietal-cell and anti-IF auto-antibodies), and transcobalamin (TC), which binds B12 to facilitate uptake by the cells [81]. Here, TCN1 is linked to vitamin B12 deficiency.