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Metachromatic leukodystrophy (MLD) is an inherited metabolic storage disorder. An inherited disorder means that the disease is caused by faulty genes passed on from parents to children. Genes are instructions that tell the body how to make all the different substances the body needs to work properly.

In MLD, the body is missing an important enzyme. Enzymes are types of proteins that break down different kinds of substances in the body. When the body is missing certain enzymes, the cells don't work properly. In MLD, the body is missing an enzyme called arylsulfatase A. The enzyme is needed to break down certain types of fat-based compounds in the body.

In patients with MLD, the body cannot break down these compounds, so they build up and can damage the brain, nervous system, bones, and other tissues.

MLD appears most often in babies and young toddlers, but it also occurs in older children and adults. MLD is rare. MLD occurs in about 1 in 40,000 to 160,000 individuals worldwide, according to the National Institutes of Health.