Metachromatic leukodystrophy (MLD) is a progressive genetic disorder affecting myelin of the central and peripheral nervous system. MLD results from an alteration in the gene for the lysosomal enzyme arylsulfatase A or its activator. Arylsulfatase A, in conjunction with its activator saposin, cleaves sulfatides from sphingoglycolipids. Defects in the process result in the accumulation of cerebroside sulfatides. This increase occurs predominantly in the nervous system resulting in dysfunction and progressive destruction of myelin. This demyelination results in the clinical syndrome of MLD.