Leukoystrophy refers to a group of disorders characterized by progressive degeneration of the white matter of the brain. The leukodystrophies are caused by imperfect growth or development of the myelin sheath, the fatty covering that acts as an insulator around nerve fibers. Myelin, which the white matter of the brain takes its colour from, is a complex substance made up of at least ten different chemicals. Each of the leukodystrophies is the result of a defect in the gene that controls the production or metabolism of one (and only one) of the component molecules of myelin.