Glial fibrillary acidic protein （GFAP） is the hallmark intermediate filament （IF; also known as nanofilament） protein in astrocytes, a main type of glial cells in the central nervous system （CNS）. Astrocytes have a range of control and homeostatic functions in health and disease. Astrocytes assume a reactive phenotype in acute CNS trauma, ischemia, and in neurodegenerative diseases. This coincides with an upregulation and rearrangement of the IFs, which form a highly complex system composed of GFAP （10 isoforms）, vimentin, synemin, and nestin [PMID: 25726916].