refers to the progressive degeneration of the white matter of the brain due to abnormalities of myelin, the fatty covering that insulates nerves. The leukodystrophies are a group of disorders caused by mutations in the genes involved in the production of myelin. Specific leukodystrophies include metachromatic leukodystrophy, Krabbe leukodystrophy, X-linked adrenoleukodystrophy, Pelizaeus-Merzbacher disease, Canavan disease, and Alexander disease. The most common symptom of a leukodystrophy is a gradual decline in functioning of an infant or child who previously appeared well. Progressive loss may appear in body tone, movements, gait, speech, ability to eat, vision, hearing, and behavior. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.