Metachromatic leukodystrophy (MLD) is a genetic disorder that affects the growth and/or development of myelin, the fatty covering which acts as an insulator around nerve fibers throughout the central and peripheral nervous systems. MLD is caused by a deficiency of the enzyme arylsulfatase A (ARSA), which leads to the toxic buildup of lipids, particularly sulfatides, in the nervous system. This buildup gradually destroys myelin-producing cells and leads to nervous system impairment. MLD has an autosomal recessive inheritance pattern, which means that both parents are carriers of the disease. There are three forms of MLD based on when the symptoms begin: late infantile, juvenile, and adult. Symptoms of MLD may include a progressive decline in mental and motor functions, muscle weakness, seizures, vision loss, and behavioral changes. There is no cure for MLD, and treatment is mainly supportive and includes therapies like medication, occupational therapy, and a feeding tube. Bone marrow transplantation may delay progression of the disease in some infantile-onset cases. The prognosis for MLD is poor, and life span varies depending on what age the condition started, but the disease course usually runs 3 to 20 years or more.