Lysosomal storage diseases are fatal group of diseases and a main application of ERT. Lysosomes are cellular organelles that are responsible for the metabolism of many different macromolecules and proteins.[5] They use enzymes to break down macromolecules, which are recycled or disposed.[5] As of 2012, there are 50 lysosomal storage diseases, and more are still being discovered.[6][5] These disorders arise because of genetic mutations that prevent the production of certain enzymes used in the lysosomes.[5] The missing enzyme often leads to a build-up of the substrate within the body. This can result in a variety of symptoms, many of which are severe and can affect the skeleton, brain, skin, heart, and the central nervous system.[6] Increasing the concentration of the missing enzyme within the body has been shown to improve the body's normal cellular metabolic processes and reduce substrate concentration in the body.[2]