Metachromatic leukodystrophy (MLD) is a disease that impairs the growth or development of the myelin sheath, the fatty covering that acts as an insulator around nerve fiber. Symptoms include hypotonia (low muscle tone), speech abnormalities and loss of mental abilities, blindness, muscle problems and dementia and symptoms can begin at any age. MLD occurs panethnically, with an estimated frequency of 1/40,000. Metachromatic leukodystrophy is inherited as an autosomal recessive trait.