GFAP and Alexander disease: Alexander disease (AxD) is a rare, fatal neurodegenerative leukodystrophy caused by heterozygous gain-of-function mutations in the GFAP gene (chromosome 17q21) encoding glial fibrillary acidic protein (GFAP), an astrocyte-specific type III intermediate filament [1]. It is characterized pathologically by the abnormal accumulation of Rosenthal fibers in astrocytes, which are ubiquitinated protein aggregates composed of GFAP, heat shock protein 27 (HSP27), and alpha B-crystallin [2].