Leukodystrophies are a group of rare, progressive, metabolic, genetic diseases that affect the brain, spinal cord and often the peripheral nerves. Each type of leukodystrophy is caused by a specific gene abnormality that leads to abnormal development or destruction of the white matter (myelin sheath) of the brain. The myelin sheath is the protective covering of the nerve and nerves can't function normally without it. Each type of leukodystrophy affects a different part of the myelin sheath, leading to a range of neurological problems.