Transferrin, a highly prevalent serum glycoprotein that functions to transport iron to tissues, is present in serum at 3–4 mg/mL and binds iron firmly yet reversibly. Transferrin fusion of peptides and proteins have been made to human transferrin to the N- and C- termini, as well as to the centrally located hinge region that links the two major lobes of transferrin together. Transferrin's N terminus is free to fuse straight through. Flexible linkers are usually employed when fusing proteins to the C terminus since it is more buried and confined by a neighboring disulfide bond. By creating peptide libraries against particular targets and combining binders from those libraries into aglyco-transferrin (N-terminal, C-terminal, loops), this capability was increased.