Metachromatic leukodystrophy (MLD) is a rare genetic disorder that affects the nervous system and leads to the destruction of the protective covering of nerve fibers called myelin. The disease is caused by a deficiency in the enzyme arylsulfatase A, which is essential for breaking down sulfatides, a type of fat molecule found in myelin. Symptoms of MLD can include muscle weakness, loss of coordination, vision and hearing loss, seizures, and developmental delays.