IDS and mucopolysaccharidosis type 2: These include the fusion of lysosomal enzymes (i.e., β-galactosidase (β-GAL), iduronate-2-sulfatase, and sulfamidase) to genetically engineered monoclonal antibody against the human transferrin receptor or the human insulin receptor for the treatment of mouse models of GM1-gangliosidosis, mucopolysaccharidosis type II (MPSII) and MPSIIIA [12,13,14].