Metachromatic leukodystrophy (MLD) is an inherited metabolic disease caused by a defect in the lysosomal enzyme arylsulfatase A (ASA), which functions to degrade sulfatides. An insufficient level of ASA causes a pathological buildup of 3-O-sulfogalactosyl ceramide (sulfatide), a sphingolipid, in, e.g., peripheral tissues, and the central nervous system (CNS). Symptoms including neurodegeneration and mental retardation appear during childhood; and early death can occur due to organ damage in the brain. Typically, treatment would include intravenous enzyme replacement therapy with recombinant ASA. However, systemically administered recombinant ASA does not cross the blood brain barrier (BBB), and therefore has little impact on the effects of the disease in the CNS.