Enzyme replacement therapy (ERT) is a medical treatment replacing an enzyme in patients in whom that particular enzyme is deficient or absent. Usually this is done by giving the patient an intravenous (IV) infusion containing the enzyme. Enzyme replacement therapy is currently available for some lysosomal diseases: Gaucher disease, Fabry disease, MPS I, MPS II (Hunter syndrome), MPS VI and Glycogen storage disease type II. Enzyme replacement therapy does not correct the underlying genetic defect, but increases the concentration of enzyme in which the patient is deficient.