The impact of astrocytes on white matter function and integrity was definitively confirmed by the identification of human white matter disorders linked to mutations in astrocyte-specific gene products such as the intermediate filament glial fibrillary acidic protein (GFAP, Alexander disease) [25] and MLC1 (megalencephalic leukoencephalopathy with subcortical cysts, MLC) [125]. Here, GFAP is linked to Alexander disease.