MLD is a rare hereditary LSD caused by deficiency of arylsulfatase A (EC 3.1.6.8) or of the Sap B. Arylsulfatase A removes the sulfate residue from sulfatide (cerebroside sulfate) in the presence of Sap B. Arylsulfatase A-deficient cells primarily accumulate the anionic glycosphingolipid sulfatide, which is a major component of myelin. The gene discussed is ARSA; the disease is metachromatic leukodystrophy.