Enzyme therapy has been used since the 1960s for the treatment of inherited metabolic diseases. These often result from the absence or lack of function of a single enzyme, and the treatment aims to replace the missing enzyme. Enzyme therapy can be applied to replace defective or absent extracellular enzymes normally secreted by cells, or specific intracellular enzymes. Replacement therapy for extracellular enzymes is relatively straightforward and includes, for example, the oral replacement of lacking digestive enzymes. This method has been adopted in the treatment of digestive disorders such as lactose intolerance. Lactose is a disaccharide sugar found in milk and is normally hydrolyzed to galactose and glucose in the intestine by the enzyme lactase. This enzyme is most active in early childhood and then decreases as a normal part of weaning and aging. Lactase deficiency can manifest as a congenital condition, where there is no...