Leukodystrophies

**Definition:** Group of rare progressive genetic diseases that are caused by mutations in genes that lead to destruction of white matter of the brain by disrupting development of the myelin sheath. More than 50 different leukodystrophies have been identified, including Alexander disease, Canavan disease, cerebrotendinous xanthomatosis, metachromatic leukodystrophy, Pelizaeus-Merzbacher disease, and Refsum disease.