X-linked adrenoleukodystrophy (ALD) is a rare genetic disorder that affects the white matter of the nervous system and the adrenal cortex. White matter is made up of nerve fibers called axons that relay nerve impulses from one cell to another. These nerve fibers are covered by myelin, an insulating layer or sheath that protects the nerve fibers. Myelin is made up of proteins and fats and gives white matter its white color. Without myelin, the signals between nerve cells cannot be transmitted properly, resulting in neurological symptoms.