A rare inherited disorder of myelin formation. Alexander disease is a progressive leukoencephalopathy whose hallmark is the widespread accumulation of cytoplasmic inclusions called Rosenthal fibers. The fibers contain GLIAL FIBRILLARY ACIDIC PROTEIN in association with ALPHA-CRYSTALLIN B CHAIN. Rosenthal fibers are found predominantly in ASTROCYTES located in the subependymal, subpial, and periventricular areas of the BRAIN.