Glial Fibrillary Acidic Protein (GFAP) is an intermediate filament protein (class III of the intermediate filament proteins) that is present in astrocytes and some ependymal cells of the central nervous system. In the peripheral nervous system, GFAP is present in Schwann cells, enteric glial cells, and satellite cells. In addition, neural stem cells strongly express GFAP. Antibodies to GFAP are very useful as markers of astrocytic cells. Many types of brain tumor, presumably derived from astrocytic cells, heavily express GFAP. GFAP is also found in the lens epithelium, Kupffer cells of the liver, in some cells in salivary tumors and has been reported in erythrocytes. Mutations in this gene cause Alexander disease, a rare disorder of astrocytes in the central nervous system. Alternative splicing of the GFAP gene results in multiple transcript variants encoding distinct isoforms. Anti-GFAP staining is useful in differentiating neoplasms of astrocyte origin from other neoplasms in the central nervous system.