The 50 kDa type III intermediate filament protein glial fibrillary acidic protein (GFAP) is a major structural component of astrocytes. GFAP associates with the calcium binding protein annexin II-p2 and S-100. Association with these proteins together with phosphorylation regulates GFAP polymerization. Astroycytes respond to brain injury by proliferatin (astrogliosis), and one of the first events to occur during astrocyte proiliferation is increased GFAP expression. Interestingly, antibodies to GFAP have been detected in individuals with dementia.