Metachromatic leukodystrophy (MLD) is one of a group of genetic disorders called the leukodystrophies. These diseases impair the growth or development of the myelin sheath, the fatty covering that acts as an insulator around nerve fibers. Myelin, which lends its color to the white matter of the brain, is a complex substance made up of varying lipids (75%) and proteins (25%). The leukodystrophies are caused by genetic defects in myelin production or metabolization of the compounds of the myelin sheath. Each of the leukodystrophies is the result of a defect in the gene that controls one (and only one) of the enzymes responsible for creating or degrading a part of the myelin. MLD is caused by a deficiency of the enzyme arylsulfatase A. MLD is one of several lipid storage diseases, which results in the toxic build-up of fatty materials (lipids) in cells in the nervous system, liver, and kidneys. There are three forms of MLD: late infantile, juvenile, and adult. Onset of the late infantile form (the most common MLD) is typically between 12 and 20 months following birth. Affected children have difficulty walking after the first year of life. Symptoms include muscle wasting and weakness, muscle rigidity, developmental delays, progressive loss of vision leading to blindness, convulsions, impaired swallowing, paralysis, and dementia. Children may become comatose. Most children with this form of MLD die by age 5. The juvenile form of MLD (between 3-10 years of age) usually begins with impaired school performance, mental deterioration, and dementia and then develop symptoms similar to the infantile form but with slower progression. The adult form commonly begins after age 16 as a psychiatric disorder or progressive dementia. Symptoms include impaired concentration, ataxia, seizures, dementia, and tremor.