Alexander disease is a rare, genetically determined degenerative disorder of the central nervous system. It is one of a group of disorders known as the leukodystrophies, diseases that result from imperfect growth or development of the myelin sheath, the fatty covering that acts as an insulator around nerve fibers in the brain. Myelin, which lends its color to the "white matter" of the brain, is a complex substance made up of at least ten different chemicals. Each of the leukodystrophies affects one (and only one) of these substances. In Alexander disease, the destruction of white matter in the brain is accompanied by the formation of fibrous protein deposits called Rosenthal fibers. It is caused by mutations in the gene for glial fibrillary acidic protein (GFAP). The majority of cases are sporadic (not inherited), but there are families in which more than one child will have the disorder. Alexander disease primarily affects males and usually begins at about 6 months of age. Symptoms may include mental and physical retardation, dementia, enlargement of the brain and head, spasticity (stiffness of arms and/or legs), and seizures. In addition to the infantile form, juvenile and adult onset forms of the disorder have been reported. These forms occur less frequently and have a longer course of progression.